Hey there, future chemists! Welcome to the exciting world of practical chemistry. Today, we're going to dive into the fundamentals of preparing some really interesting inorganic compounds, specifically focusing on two superstars: Mohr's salt and Potash alum.

Before we get into the specifics, let's understand what "preparation" in chemistry really means.

### 1. What is "Preparation" in Chemistry?

Think of it like cooking! When you prepare a dish, you take different ingredients, mix them in specific proportions, and follow a recipe to create something new and delicious. In chemistry, it's pretty similar! We take different chemical substances (our 'ingredients'), combine them under controlled conditions (our 'recipe'), and aim to create a new, desired chemical compound.

The goal isn't just to mix things randomly, though. We want to:

• Create a pure substance.


• Ensure it has the correct chemical composition.


• Sometimes, we prepare compounds to study their properties or use them for specific applications.

### 2. What are Inorganic Compounds?

You've probably heard of 'organic' and 'inorganic' chemistry. Organic compounds are generally those that contain carbon atoms bonded to hydrogen atoms, forming the backbone (like in plastics, fuels, and living organisms). Inorganic compounds, on the other hand, are pretty much all other compounds. They don't typically have the carbon-hydrogen backbone.

Examples of inorganic compounds include salts (like common table salt, NaCl), acids (like sulfuric acid, H₂SO₄), bases (like sodium hydroxide, NaOH), and many minerals. The compounds we're preparing today, Mohr's salt and Potash alum, definitely fall into the inorganic category.

### 3. Understanding the "Building Blocks": Single Salts, Double Salts, and Complex Compounds

This is a crucial concept to grasp before we move to Mohr's salt and Potash alum. Imagine you have different types of LEGO bricks.

* Single Salts: These are like basic LEGO bricks, each a complete unit on its own. For example, Ferrous Sulfate (FeSO₄) is a single salt. Ammonium Sulfate ((NH₄)₂SO₄) is another single salt. When dissolved in water, they simply break down into their respective ions (e.g., FeSO₄ gives Fe²⁺ and SO₄²⁻).

* Double Salts: Now, imagine taking two *different types* of single LEGO bricks and snapping them together in a very specific way to form a larger, stable structure. This new structure is a double salt! The key characteristic of double salts is that while they are stable as solids, when you dissolve them in water, they completely break down (dissociate) into all the individual ions that made them up.
* Our two main compounds for today, Mohr's salt and Potash alum, are perfect examples of double salts. They are formed from two different simple salts, crystallizing together in a fixed stoichiometric ratio with a definite crystal structure.
* For example, Mohr's salt is made from Ferrous Sulfate and Ammonium Sulfate. When it dissolves, you get Fe²⁺ ions, NH₄⁺ ions, and SO₄²⁻ ions – all the ions from the original single salts!

* Complex Compounds (Brief Mention for Contrast): What if you took those LEGO bricks and glued them together so strongly that even if you threw them in water, some parts would *not* separate? That's closer to a complex compound. In solution, complex compounds *do not* dissociate completely into all their constituent ions. They maintain a "complex ion" which behaves as a single unit. For instance, in K₄[Fe(CN)₆], when dissolved, you get K⁺ ions and a [Fe(CN)₆]⁴⁻ complex ion, not separate Fe²⁺ and CN⁻ ions. This is the fundamental difference from double salts.

Feature	Double Salts	Complex Compounds
Nature in Solid State	Stable crystalline compounds formed from two simple salts.	Stable crystalline compounds, often containing a central metal atom/ion bonded to ligands.
Behavior in Solution	Dissociate completely into all constituent simple ions.	Do not dissociate completely; maintain a complex ion.
Test for Ions	Gives tests for all original simple ions (e.g., Fe²⁺, NH₄⁺, SO₄²⁻).	Does not give tests for all original simple ions (e.g., K₄[Fe(CN)₆] won't test for Fe²⁺ or CN⁻ independently).
Example	Mohr's salt (FeSO₄.(NH₄)₂SO₄.6H₂O)	Potassium ferrocyanide (K₄[Fe(CN)₆])

### 4. Meet Our Stars: Mohr's Salt and Potash Alum

Now that we understand double salts, let's get acquainted with our specific compounds.

#### a. Mohr's Salt (Ferrous Ammonium Sulfate)

* Formula: FeSO₄.(NH₄)₂SO₄.6H₂O
* What it is: This is a beautiful light green crystalline solid. Notice the '6H₂O' – this means it's a hydrated salt, and these six water molecules are an essential part of its crystal structure. We call this "water of crystallization."
* Why it's special: If you just use Ferrous Sulfate (FeSO₄) solution, the Fe²⁺ ions are very easily oxidized by air to Fe³⁺ ions. This makes pure FeSO₄ solution unstable for many experiments. But in Mohr's salt, the presence of ammonium sulfate and its stable crystalline structure helps protect the Fe²⁺ ions from oxidation, making it a much more stable and reliable source of Fe²⁺.
* Applications: It's super important in quantitative analysis, especially in titrations, where it acts as a primary standard – a substance of known purity and concentration used to standardize other solutions.

#### b. Potash Alum (Potassium Aluminium Sulfate)

* Formula: K₂SO₄.Al₂(SO₄)₃.24H₂O
* What it is: This compound forms large, colorless, transparent octahedral crystals. Again, notice the '24H₂O' – a significant amount of water of crystallization!
* Why it's special: Potash alum is a classic example of an 'alum', which is a specific class of double sulfates. It contains both potassium sulfate (a single salt) and aluminum sulfate (another single salt).
* Applications: You might actually have encountered this without realizing it! Potash alum is widely used for:

• Water purification: It acts as a coagulant (flocculant), making tiny suspended impurities in water clump together so they can be easily removed.


• Dyeing: It helps dyes stick to fabrics (as a mordant).


• Tanning leather: It's used in the processing of leather.


• Paper sizing, fireproofing, and even in some aftershaves!

### 5. The Basic Chemistry Involved in Preparation: The Art of Crystallization

The core chemistry behind preparing these double salts like Mohr's salt and Potash alum revolves around a technique called crystallization. It's not just about mixing; it's about getting the compound to form pure, stable crystals.

Here's a simplified breakdown of the general steps involved:

1. Dissolving the Individual Salts:
* First, we take the individual single salts that make up our double salt (e.g., ferrous sulfate and ammonium sulfate for Mohr's salt; potassium sulfate and aluminum sulfate for Potash alum).
* We dissolve each of these salts in a minimal amount of hot water. Using hot water usually helps dissolve more solute.
* Important Note: For Mohr's salt, we often add a little dilute sulfuric acid (H₂SO₄) to the solution. Why? To prevent the hydrolysis of FeSO₄ (which can make the solution cloudy) and to suppress the oxidation of Fe²⁺ to Fe³⁺ by air, thereby enhancing the stability of the Fe²⁺ ions even before crystallization.

2. Mixing the Solutions:
* Once both individual salts are dissolved, we mix their hot, saturated solutions in a specific stoichiometric ratio (meaning, the correct proportions required by the chemical formula).
* For Mohr's salt, it's typically a 1:1 molar ratio of FeSO₄ to (NH₄)₂SO₄.
* For Potash alum, it's a 1:1 molar ratio of K₂SO₄ to Al₂(SO₄)₃.

3. Concentration (Optional but Common):
* Sometimes, the mixed solution might not be concentrated enough to crystallize readily. In such cases, we gently heat the mixed solution to evaporate some of the water. This increases the concentration of the dissolved salts, making the solution 'supersaturated'.
* Remember: Supersaturated means the solution contains more dissolved solute than it normally would at that temperature. It's a bit unstable, just waiting for a reason to crystallize!

4. Slow Cooling and Crystallization:
* This is the most critical step! We allow the hot, concentrated (or supersaturated) solution to cool down, preferably slowly and undisturbed, usually at room temperature.
* As the solution cools, the solubility of the double salt decreases. The dissolved ions then start coming together in a highly ordered fashion, arranging themselves to form the beautiful, defined crystal structure of the double salt.
* Tip: Slow cooling generally promotes the formation of larger, purer crystals. Rapid cooling often leads to smaller, less pure crystals.

5. Filtration:
* Once crystals have formed, we separate them from the remaining liquid, which is called the 'mother liquor', using filtration.

6. Washing:
* The freshly filtered crystals might have some impurities adhering to their surface from the mother liquor. We gently wash them with a small amount of a suitable solvent (often a cold, dilute solution of the desired product or a mixture of alcohol/water) to remove these surface impurities.
* Caution: Don't use too much solvent, or you might redissolve your precious crystals!

7. Drying:
* Finally, the washed crystals are dried, usually by pressing them gently between filter papers or placing them in a desiccator, to remove any surface moisture. It's important not to overheat them, especially for hydrated salts, as they can lose their water of crystallization and decompose.

### 6. The Magic of Water of Crystallization

As you saw in the formulas (6H₂O for Mohr's salt, 24H₂O for Potash alum), these compounds incorporate water molecules directly into their crystal lattice. This isn't just adsorbed water; these water molecules are essential for the crystal's structure, shape, and stability. Losing this water (e.g., by strong heating) would change the compound entirely, often turning it into an amorphous powder and altering its properties.

### 7. Why is Crystallization so Important?

Beyond just forming pretty crystals, crystallization is a powerful purification technique. When a compound crystallizes from a solution, the molecules arrange themselves very specifically. Impurity molecules often don't 'fit' into this perfect arrangement and are left behind in the mother liquor. This means you end up with a much purer product!

So, the next time you see a beautiful crystal, remember the intricate dance of atoms and molecules that came together, often through the process of crystallization, to form that perfect structure. This is the fundamental chemistry that makes the preparation of compounds like Mohr's salt and Potash alum possible!

Welcome, future scientists! Today, we're taking a deep dive into the fascinating world of inorganic compound preparation, specifically focusing on two very important double salts: Mohr's Salt and Potash Alum. These aren't just academic exercises; understanding their synthesis reveals core principles of solubility, acid-base chemistry, redox reactions, and crystallization – all crucial for your JEE journey.

Before we jump into the specifics, let's quickly refresh our understanding of different types of salts.

1. Simple Salts: Formed by the complete neutralization of an acid by a base (e.g., NaCl, K₂SO₄, FeSO₄).


2. Double Salts: These are addition compounds formed by the crystallization of two different simple salts from a solution in stoichiometric proportions. Crucially, in aqueous solution, they dissociate completely into their constituent ions. They lose their individual identity and characteristics in solution (e.g., Mohr's salt, Potash alum).


3. Complex Salts (Coordination Compounds): These are also addition compounds, but in solution, they do not dissociate into all constituent ions. They form complex ions which retain their identity (e.g., K₄[Fe(CN)₆] – in solution, it gives K⁺ and [Fe(CN)₆]⁴⁻, not Fe²⁺ and CN⁻ ions).

Mohr's salt and Potash alum fall squarely into the category of double salts. Let's explore their preparation in detail.

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### 1. Preparation of Mohr's Salt (Ferrous Ammonium Sulfate Hexahydrate)

Formula: (NH₄)₂Fe(SO₄)₂·6H₂O
IUPAC Name: Ammonium ferrous sulfate hexahydrate

Mohr's salt is a pale green crystalline solid. Its preparation is a classic experiment, not just for synthesizing a compound, but also for understanding crucial chemical principles like preventing oxidation and hydrolysis.

#### Why is it important? (JEE Focus)
Mohr's salt is widely used as a primary standard in volumetric analysis, particularly in redox titrations (e.g., against potassium permanganate, KMnO₄).
What makes it a good primary standard?

• It can be obtained in a high state of purity.


• It is stable in air and non-hygroscopic (does not absorb moisture from the air), unlike simple ferrous sulfate (FeSO₄·7H₂O), which is prone to efflorescence and oxidation.


• It has a high equivalent weight, which reduces weighing errors.


• Its aqueous solutions are fairly stable over short periods, especially in the presence of dilute acid.

#### Chemistry Involved in Preparation:

The synthesis of Mohr's salt involves the crystallization of an equimolar mixture of ferrous sulfate and ammonium sulfate from an acidic aqueous solution.

Reactants:
1. Ferrous Sulfate Heptahydrate (FeSO₄·7H₂O): A common source of Fe²⁺ ions. This compound is often impure and susceptible to oxidation.
2. Ammonium Sulfate ((NH₄)₂SO₄): Provides NH₄⁺ and SO₄²⁻ ions.
3. Dilute Sulfuric Acid (H₂SO₄): This is a critical component, playing multiple roles.

Overall Reaction:
FeSO₄·7H₂O(aq) + (NH₄)₂SO₄(aq) → (NH₄)₂Fe(SO₄)₂·6H₂O(s) + H₂O

Let's break down the roles of each component and the underlying chemistry:

1. 
   Formation of a Double Salt:
   When solutions of ferrous sulfate and ammonium sulfate are mixed in equimolar proportions and allowed to crystallize, the double salt, Mohr's salt, is formed. The crystal lattice of Mohr's salt is more stable than that of individual ferrous sulfate.
   
   
   Equation for Dissolution and Recombination:
   
   
   FeSO₄(aq) + (NH₄)₂SO₄(aq) $xrightarrow{ ext{Crystallization}}$ (NH₄)₂Fe(SO₄)₂·6H₂O(s)
   


2. 
   Role of Dilute Sulfuric Acid (H₂SO₄): (JEE Advanced Concept)
   This is perhaps the most important chemical aspect of the preparation. A small amount of dilute H₂SO₄ is added to the solution of the mixed salts.
   
   
   
   • 
     Prevention of Hydrolysis of Fe²⁺ Ions:
     Iron(II) ions in aqueous solution are acidic and can undergo hydrolysis, especially at higher temperatures or in neutral solutions, leading to the precipitation of iron(II) hydroxide.
     Fe²⁺(aq) + 2H₂O(l) ⇌ Fe(OH)₂(s) + 2H⁺(aq)
     By adding dilute H₂SO₄, the concentration of H⁺ ions in the solution increases. According to Le Chatelier's principle, this shifts the equilibrium to the left, preventing the precipitation of Fe(OH)₂. This ensures that Fe²⁺ ions remain in solution.
     
   
   
   • 
     Prevention of Oxidation of Fe²⁺ to Fe³⁺:
     Ferrous ions (Fe²⁺) are easily oxidized to ferric ions (Fe³⁺) by atmospheric oxygen, particularly in neutral or alkaline solutions.
     4Fe²⁺(aq) + O₂(g) + 2H₂O(l) → 4Fe³⁺(aq) + 4OH⁻(aq)
     The presence of H⁺ ions (from H₂SO₄) suppresses the formation of OH⁻ ions. In an acidic medium, the redox potential for the oxidation of Fe²⁺ is higher, making it less prone to oxidation.
     
     
     Redox Half-Reactions:
     O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) $quad E^circ = +1.23 ext{ V}$
     Fe²⁺(aq) → Fe³⁺(aq) + e⁻ $quad E^circ = -0.77 ext{ V}$
     The presence of H⁺ shifts the oxygen reduction potential higher, making the overall reaction less favorable for Fe²⁺ oxidation compared to neutral conditions. If Fe³⁺ forms, it would precipitate as reddish-brown Fe(OH)₃, contaminating the product.
     
   
   
   
   


3. 
   Crystallization:
   Mohr's salt is less soluble than the individual simple salts at lower temperatures. As the hot, concentrated solution is cooled slowly, the solubility limit is exceeded, and the Mohr's salt crystallizes out, forming well-defined green crystals. Slow cooling promotes the formation of larger, purer crystals.
   

#### Practical Steps (Deep Dive into Laboratory Procedure):

1. Preparation of Solutions:
* Weigh out accurately an equimolar amount of FeSO₄·7H₂O and (NH₄)₂SO₄. (e.g., If you take 27.8g of FeSO₄·7H₂O (0.1 mol), you'd take 13.2g of (NH₄)₂SO₄ (0.1 mol)).
* Dissolve both salts separately in minimum quantities of warm water. Add a few drops of dilute H₂SO₄ to the ferrous sulfate solution to prevent hydrolysis and oxidation.
2. Mixing and Heating:
* Mix the two solutions. Add about 1-2 mL of dilute H₂SO₄ to the combined solution.
* Heat the mixture gently on a hot plate or water bath to obtain a clear, concentrated solution. This is done to ensure complete dissolution and to increase the concentration for crystallization.
3. Hot Filtration (Optional but Recommended for Purity):
* If any undissolved impurities or reddish-brown Fe(OH)₃ (due to oxidation) are present, filter the hot solution through a fluted filter paper into a clean beaker. This removes insoluble impurities before crystallization.
4. Cooling and Crystallization:
* Allow the clear, hot filtrate to cool slowly and undisturbed. This is crucial for forming large, well-shaped crystals. You can place the beaker in a larger container filled with lukewarm water, and allow it to cool gradually to room temperature, then potentially in an ice bath for further yield.
5. Washing the Crystals:
* Once crystals have formed, decant the mother liquor (the solution remaining after crystallization).
* Wash the crystals with a small amount of cold dilute H₂SO₄. Why not plain water? Washing with plain water would cause some of the Mohr's salt to dissolve (due to its solubility in water) and could also promote hydrolysis of Fe²⁺. The dilute acid prevents both.
* Finally, wash once quickly with a small amount of cold alcohol to remove surface moisture and traces of acid, as Mohr's salt is insoluble in alcohol.
6. Drying:
* Press the crystals gently between folds of filter paper to dry them. Avoid heating to dry, as it can cause efflorescence (loss of water of crystallization) or decomposition.

Parameter	Significance in Mohr's Salt Preparation	JEE Relevance
Equimolar Amounts	Ensures stoichiometric combination for double salt formation.	Stoichiometry, limiting reagent concepts.
Dilute H₂SO₄	Prevents hydrolysis of Fe²⁺ and oxidation to Fe³⁺.	Le Chatelier's Principle, Redox chemistry, Stability of ions.
Slow Cooling	Promotes growth of larger, purer crystals.	Crystallization kinetics, purity.
Washing with cold dilute H₂SO₄	Minimizes dissolution of product and prevents hydrolysis.	Solubility, practical techniques.
Non-hygroscopic nature	Key property making it a primary standard.	Definition of primary standard.

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### 2. Preparation of Potash Alum (Potassium Aluminum Sulfate Dodecahydrate)

Formula: KAl(SO₄)₂·12H₂O (or K₂SO₄·Al₂(SO₄)₃·24H₂O)
IUPAC Name: Potassium aluminum sulfate dodecahydrate

Potash alum is a colorless, crystalline solid. It belongs to a class of double salts called 'alums', which have the general formula M¹⁺M³⁺(SO₄)₂·12H₂O, where M¹⁺ is a monovalent cation (like K⁺, Na⁺, NH₄⁺) and M³⁺ is a trivalent cation (like Al³⁺, Fe³⁺, Cr³⁺).

#### Why is it important? (JEE Focus)
Potash alum has numerous industrial and laboratory applications:

• Water Purification: As a coagulant, it causes suspended impurities to aggregate and settle.


• Mordant in Dyeing: Helps dyes to adhere to fabrics.


• Paper Industry: Sizing agent.


• Antiseptic: Used in aftershave lotions.

#### Chemistry Involved in Preparation:

Potash alum can be prepared by the crystallization of an equimolar mixture of potassium sulfate and aluminum sulfate from an acidic aqueous solution. A common laboratory method starts from aluminum metal or aluminum hydroxide, which is then converted into aluminum sulfate.

Reactants:
1. Aluminum metal (Al) or Aluminum hydroxide (Al(OH)₃): Source of Al³⁺ ions.
2. Potassium Hydroxide (KOH): Used to convert Al to soluble aluminate.
3. Sulfuric Acid (H₂SO₄): Provides sulfate ions and creates an acidic environment.
4. Potassium Sulfate (K₂SO₄): Provides K⁺ and additional SO₄²⁻ ions.

#### Method from Aluminum Metal (Common Lab Method - Deep Dive):

This method highlights the amphoteric nature of aluminum.

Step 1: Conversion of Aluminum Metal to Soluble Potassium Tetrahydroxoaluminate(III)
Aluminum is an amphoteric metal, meaning it reacts with both acids and bases. Here, it reacts with a strong base like KOH to form a soluble complex salt.
2Al(s) + 2KOH(aq) + 6H₂O(l) → 2K[Al(OH)₄](aq) + 3H₂(g)
(Potassium tetrahydroxoaluminate(III))
Important: The reaction is exothermic. Heating helps dissolve aluminum. An excess of KOH ensures all aluminum reacts and the aluminate remains soluble.

Step 2: Neutralization and Formation of Aluminum Sulfate
The soluble potassium tetrahydroxoaluminate(III) is then acidified with dilute sulfuric acid. The acid first neutralizes the excess KOH, then reacts with the aluminate to form aluminum hydroxide, which then redissolves in excess acid to form aluminum sulfate.
K[Al(OH)₄](aq) + H₂SO₄(aq) → K₂SO₄(aq) + Al₂(SO₄)₃(aq) + H₂O(l) (Simplified overall)

Let's look at the intermediate steps:
K[Al(OH)₄](aq) + H₂SO₄(aq) → Al(OH)₃(s) + K₂SO₄(aq) + H₂O(l) (initially Al(OH)₃ precipitates)
2Al(OH)₃(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 6H₂O(l) (Al(OH)₃ redissolves in excess acid)
Critical Point: Enough H₂SO₄ must be added to ensure all aluminum is converted to Al₂(SO₄)₃ and the solution is acidic. An acidic medium prevents the hydrolysis of Al³⁺ ions, which would lead to the precipitation of Al(OH)₃.

Step 3: Formation of Potash Alum
The solutions of potassium sulfate (formed in Step 2) and aluminum sulfate are then combined. Upon cooling, potash alum crystallizes out.
K₂SO₄(aq) + Al₂(SO₄)₃(aq) + 12H₂O(l) → KAl(SO₄)₂·12H₂O(s)

#### Practical Steps (Deep Dive into Laboratory Procedure):

1. Dissolving Aluminum:
* Weigh out a known quantity of clean aluminum scrap (foil/turnings).
* Place the aluminum in a beaker and add a concentrated solution of KOH (e.g., 20% w/v).
* Heat gently on a hot plate or water bath until all the aluminum dissolves, producing hydrogen gas (perform in a fume hood!).
* Keep adding small amounts of hot water if the solution becomes too concentrated during heating.
2. Filtration:
* Filter the hot solution (K[Al(OH)₄]) through a funnel with a cotton plug or fluted filter paper to remove any insoluble impurities (e.g., unreacted Al, carbonaceous matter).
3. Acidification and Formation of Al₂ (SO₄)₃:
* Carefully add dilute H₂SO₄ to the filtrate while stirring continuously. Initially, white gelatinous Al(OH)₃ will precipitate.
* Continue adding H₂SO₄ until the precipitated Al(OH)₃ redissolves completely, forming a clear solution. This ensures all aluminum is present as Al₂(SO₄)₃ and the solution is sufficiently acidic. Test with litmus paper (should be acidic).
* The formation of Al(OH)₃ and its redissolution is a key step demonstrating the amphoteric nature of aluminum and the importance of maintaining an acidic environment.
4. Adding Potassium Sulfate:
* In another beaker, dissolve a calculated amount of K₂SO₄ (typically in a 1:1 molar ratio with Al₂(SO₄)₃ generated) in warm water.
* Mix the K₂SO₄ solution with the acidic aluminum sulfate solution.
5. Concentration and Crystallization:
* Heat the combined solution gently to concentrate it until a thin film appears on the surface when a glass rod is dipped and blown upon. This indicates supersaturation.
* Allow the solution to cool slowly and undisturbed. Large, clear, octahedral crystals of potash alum will form. Slow cooling is essential for good crystal growth.
6. Washing and Drying:
* Separate the crystals by decantation or filtration.
* Wash the crystals with a small amount of cold water. Alums are relatively stable and less prone to hydrolysis in water compared to simple Al salts. Washing with cold water removes surface impurities and mother liquor.
* Dry the crystals by pressing them gently between filter papers or by air-drying at room temperature. Avoid heating, as alums lose their water of crystallization upon heating ("burnt alum").

Parameter	Significance in Potash Alum Preparation	JEE Relevance
Reaction of Al with KOH	Demonstrates amphoteric nature of Al, forms soluble aluminate complex.	Amphoteric compounds, complex formation.
Addition of H₂SO₄	Neutralizes aluminate, forms Al₂(SO₄)₃, prevents Al(OH)₃ precipitation (hydrolysis of Al³⁺).	Acid-base reactions, solubility, hydrolysis of metal ions.
Slow Cooling	Promotes growth of large, octahedral crystals.	Crystal geometry, crystallization process.
12 Water Molecules	Characteristic of alums, contributing to crystal structure and properties.	Hydration, crystal lattice.
Behavior on Heating	Loses water of crystallization (efflorescence/decomposition) to form 'burnt alum'.	Thermal stability, water of crystallization.

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### Conclusion (JEE Focus)

Both Mohr's salt and Potash alum are excellent examples of double salts, which retain the properties of their constituent ions in solution. Their preparation illustrates key concepts:
* Solubility and Crystallization: How different solubilities at varying temperatures drive crystal formation.
* Acid-Base Chemistry: The critical role of acid in preventing undesirable reactions like hydrolysis (Fe²⁺ and Al³⁺) or precipitation.
* Redox Chemistry: Preventing oxidation of Fe²⁺ in Mohr's salt.
* Amphoteric Nature: Demonstrated by aluminum in alum preparation.
* Primary Standards: Understanding why specific compounds are chosen for analytical purposes.
* Stoichiometry: Importance of reacting components in correct proportions.

Mastering these preparations involves not just memorizing steps but deeply understanding the chemical reasons behind each stage. Keep practicing and relating these concepts to other areas of inorganic chemistry!

Mastering the preparation of inorganic compounds like Mohr's salt and Potash Alum is crucial for both theoretical understanding and practical applications in the lab. Here are some mnemonics and short-cuts to help you remember the key aspects efficiently.

1. Mohr's Salt (Ferrous Ammonium Sulfate Hexahydrate)

Mohr's salt is a double salt with the formula (NH₄)₂Fe(SO₄)₂·6H₂O. It's often used as a primary standard in titrations due to its stability against oxidation.

• Formula Mnemonic:
  
  
  
  • "Mohr's Two Feels Two-rific with Six drinks."
  
  
  • Mohr's: Refers to Mohr's Salt.
  
  
  • Two Fe: Indicates 2 (NH₄) and 1 Fe.
  
  
  • Two-rific: Indicates 2 (SO₄).
  
  
  • Six drinks: Represents 6 H₂O molecules.
  
  
  • Putting it together: (NH₄)₂Fe(SO₄)₂·6H₂O
  
  
  
  


• Preparation Mnemonic (Key Reagents):
  
  
  
  • "For All Students, Mohr's Salt is An Acidic Solution."
  
  
  • For (FeSO₄): Ferrous Sulfate.
  
  
  • All (Ammonium): Ammonium Sulfate ((NH₄)₂SO₄).
  
  
  • Students (Sulfuric): Sulfuric Acid (H₂SO₄ – added to prevent hydrolysis/oxidation of Fe²⁺).
  
  
  • Mohr's Salt is the product.
  
  
  • An Acidic Solution: Emphasizes the acidic medium for preparation.
  
  
  • Overall Reaction Concept: Equimolar solutions of ferrous sulfate and ammonium sulfate are mixed in the presence of dilute sulfuric acid, followed by crystallization.
  
  
  
  

2. Potash Alum (Potassium Aluminium Sulfate Dodecahydrate)

Potash Alum is another double salt, commonly known simply as "Alum." Its formula is KAl(SO₄)₂·12H₂O (or K₂SO₄·Al₂(SO₄)₃·24H₂O in its full form, where 24H₂O corresponds to 2 * 12H₂O for 2 KAl(SO₄)₂ units).

• Formula Mnemonic (KAl(SO₄)₂·12H₂O):
  
  
  
  • "King Alexander's Two sulfate armies drink Twelve waters."
  
  
  • King: Represents K (Potassium).
  
  
  • Alexander's: Represents Al (Aluminium).
  
  
  • Two sulfate armies: Represents 2 (SO₄).
  
  
  • Twelve waters: Represents 12 H₂O.
  
  
  • Putting it together: KAl(SO₄)₂·12H₂O
  
  
  
  


• Preparation Mnemonic (Key Reagents):
  
  
  
  • "Kids Always Start Alums by Watering."
  
  
  • Kids (K₂SO₄): Potassium Sulfate.
  
  
  • Always (Al₂(SO₄)₃): Aluminium Sulfate.
  
  
  • Start Alums: Refers to the formation of Alum.
  
  
  • Watering (H₂O): Indicates crystallization from an aqueous solution.
  
  
  • Overall Reaction Concept: Equimolar solutions of potassium sulfate and aluminium sulfate are mixed, and the resulting solution is allowed to crystallize.
  
  
  
  

JEE Specific Tip: Remember that both Mohr's salt and Potash Alum are examples of double salts, which retain their identity only in the solid state. In aqueous solutions, they dissociate completely into their constituent ions.

Keep practicing these mnemonics to solidify your memory of these important compounds and their preparations!

Here are some quick tips focusing on the chemistry involved in the preparation of inorganic compounds like Mohr’s salt and potash alum, crucial for both CBSE and JEE exams.

General Principles for Inorganic Preparations

• Stoichiometry is Key: Always use reactants in appropriate stoichiometric ratios or with a slight excess of the more volatile/cheaper reactant.


• Solubility: Understand the solubility profiles of reactants and products in different solvents (usually water). This guides dissolution and crystallization steps.


• Crystallization: Most pure inorganic compounds are obtained by slow crystallization from saturated solutions. Slow cooling promotes larger, purer crystals.


• Purity: Recrystallization is a common method for purification. Washing crystals with a suitable solvent (e.g., alcohol to remove water without dissolving the salt) helps remove surface impurities.


• Yield: Factors affecting yield include complete reaction, efficient crystallization, and minimal loss during filtration and washing.

Mohr’s Salt (Ferrous Ammonium Sulfate, FAS)

Formula: (NH₄)₂Fe(SO₄)₂·6H₂O

• Nature: It is a double salt, not a complex salt. It dissociates completely into Fe²⁺, NH₄⁺, and SO₄^2- ions in aqueous solution.


• Reactants: Equimolar solutions of ferrous sulfate (FeSO₄·7H₂O) and ammonium sulfate ((NH₄)₂SO₄) are mixed.


• Role of Sulfuric Acid (H₂SO₄):
  
  
  
  • A small amount of dilute H₂SO₄ is added to the solution before crystallization.
  
  
  • Crucial Tip: It prevents the hydrolysis of Fe²⁺ ions (Fe²⁺ + 2H₂O ⇌ Fe(OH)₂ + 2H⁺) and, more importantly, slows down the oxidation of Fe²⁺ to Fe³⁺ by atmospheric oxygen, which occurs more readily in neutral or alkaline solutions.
  
  
  • Maintains an acidic medium, which is favorable for crystallization and stability.
  
  
  
  


• Crystallization: The solution is concentrated by heating and then allowed to cool slowly to obtain light green, monoclinic crystals.


• Stability: Mohr's salt is more stable to oxidation than simple ferrous sulfate due to the presence of ammonium sulfate, which stabilizes the Fe²⁺ ion in the crystal lattice.

Potash Alum (Potassium Aluminum Sulfate)

Formula: KAl(SO₄)₂·12H₂O

• Nature: It is also a double salt and belongs to the general class of 'alums' (M^IM^III(SO₄)₂·12H₂O, where M^I is a monovalent cation like K⁺ and M^III is a trivalent cation like Al³⁺).


• Common Preparation Route:
  
  
  
  • Often starts with aluminum scrap or aluminum hydroxide (Al(OH)₃).
  
  
  • Aluminum metal is reacted with dilute H₂SO₄ to form Al₂(SO₄)₃.
    
    2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)
  
  
  • The aluminum sulfate solution is then mixed with a concentrated solution of potassium sulfate (K₂SO₄).
  
  
  
  


• Crystal Formation: The mixed solution is heated to concentrate and then allowed to cool slowly. Colorless, octahedral crystals of potash alum separate out.


• Solubility: Potash alum is highly soluble in hot water and less soluble in cold water, making crystallization an effective purification method.

Exam Focus (JEE & CBSE)

• Mohr’s Salt: Focus on its double salt nature, the role of H₂SO₄ (preventing oxidation of Fe²⁺ and hydrolysis), and its use in titrations.


• Potash Alum: Understand its general formula, double salt nature (specifically, an alum), and common starting materials (Al, Al(OH)₃). Recognize its uses (water purification, dyeing).


• Key Reactions: Be familiar with the basic reactions involved in the formation of the constituent sulfates for both compounds.

Mastering these details will help you confidently tackle questions related to the preparation and properties of these important inorganic compounds.

Welcome to the 'Intuitive Understanding' section! Here, we'll demystify the core chemical principles behind the preparation of inorganic compounds like Mohr's salt and potash alum, focusing on the 'why' behind the 'how'.

Understanding Double Salts

Both Mohr's salt and potash alum are excellent examples of double salts. An intuitive way to understand them is to think of them as two simple salts that decide to "hold hands" and crystallize together in a fixed stoichiometric ratio with water molecules, forming a new, stable crystal lattice.

• Distinct from Complex Salts (JEE Focus): While they crystallize as a single entity, in aqueous solution, double salts completely dissociate into their constituent ions. For example, Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O) in water will give Fe²⁺, NH₄⁺, and SO₄²⁻ ions. A complex salt, like K₄[Fe(CN)₆], would dissociate into K⁺ and the complex ion [Fe(CN)₆]⁴⁻, where the Fe-CN bonds remain intact. This fundamental difference is often tested in JEE.


• Stability: The formation of a double salt is often energetically favorable, leading to a more stable crystal structure in the solid state compared to the individual salts.


• Water of Crystallization: The presence of a definite number of water molecules (e.g., 6 in Mohr's salt, 12 in potash alum) is crucial. These water molecules are integral to the crystal lattice, stabilizing the structure and influencing its shape and properties.

Chemistry of Mohr's Salt Preparation

Mohr's salt (Ferrous Ammonium Sulfate, (NH₄)₂Fe(SO₄)₂·6H₂O) is prepared by crystallizing equimolar solutions of ferrous sulfate (FeSO₄) and ammonium sulfate ((NH₄)₂SO₄) together.

• Why Ferrous Sulfate? We need Fe²⁺ ions. Iron reacts with dilute sulfuric acid to form ferrous sulfate.


• Preventing Oxidation & Hydrolysis: When preparing FeSO₄ solution, a little dilute sulfuric acid is added.
  
  
  
  • Oxidation: Fe²⁺ ions are easily oxidized to Fe³⁺ by atmospheric oxygen. The acidic medium (H⁺ ions) helps to retard this oxidation.
  
  
  • Hydrolysis: Fe²⁺ ions can undergo hydrolysis to form insoluble Fe(OH)₂ (which further oxidizes to Fe(OH)₃). Acid suppresses this hydrolysis by shifting the equilibrium.
  
  
  
  


• Crystallization Principle:
  
  
  
  1. Dissolution: The two simple salts are dissolved in hot water. Hot water increases solubility, allowing more salt to dissolve.
  
  
  2. Concentration: The solution is then concentrated by evaporation. As water evaporates, the concentration of the salts increases.
  
  
  3. Cooling: Upon cooling, the solubility of the salts decreases, leading to supersaturation. The excess dissolved salt then separates out as pure crystals of Mohr's salt. This process also helps in purifying the product as impurities often have different solubilities and remain in the mother liquor.
  
  
  
  

Chemistry of Potash Alum Preparation

Potash alum (Potassium Aluminium Sulfate, KAl(SO₄)₂·12H₂O) is a type of alum (M¹⁺M³⁺(SO₄)₂·12H₂O) and is prepared from potassium sulfate (K₂SO₄) and aluminium sulfate (Al₂(SO₄)₃).

• Source of Aluminium: Often, scrap aluminium (Al) is used.
  
  
  
  1. Reaction with KOH: Aluminium reacts with potassium hydroxide (KOH) solution to form potassium meta-aluminate (KAlO₂). This is an amphoteric nature of aluminium.
     

     2Al(s) + 2KOH(aq) + 2H₂O(l) → 2KAlO₂(aq) + 3H₂(g)

     
     
  
  
  2. Acidification: The potassium meta-aluminate solution is then acidified with dilute sulfuric acid (H₂SO₄). This converts KAlO₂ into aluminium sulfate (Al₂(SO₄)₃) and potassium sulfate (K₂SO₄).
     

     2KAlO₂(aq) + 4H₂SO₄(aq) → Al₂(SO₄)₃(aq) + K₂SO₄(aq) + 4H₂O(l)

     
     
  
  
  
  


• Crystallization of Alum: The solution now contains Al₂(SO₄)₃ and K₂SO₄. Similar to Mohr's salt, this solution is concentrated and cooled to obtain beautiful, large crystals of potash alum, which is also a double salt with 12 molecules of water of crystallization. The presence of these 12 water molecules ensures the characteristic octahedral shape of alum crystals.

In essence, the preparation of these inorganic compounds boils down to carefully controlling solubility, pH, and crystallization conditions to allow two simple salts to combine into a more stable, crystalline double salt.

Real World Applications of Inorganic Compounds, Mohr's Salt, and Potash Alum

Understanding the chemistry involved in the preparation of inorganic compounds, including specific examples like Mohr's salt and potash alum, provides insights into their diverse and critical real-world applications across various industries and daily life.

1. General Inorganic Compounds

Inorganic compounds, prepared through various chemical processes, form the backbone of numerous industries:

• Chemical Industry: Many basic inorganic chemicals like sulfuric acid (H₂SO₄), ammonia (NH₃), sodium hydroxide (NaOH), and chlorine (Cl₂) are fundamental raw materials for synthesizing a vast array of products, from fertilizers and plastics to pharmaceuticals and detergents.


• Agriculture: Fertilizers (e.g., urea, ammonium nitrate, superphosphates) are inorganic compounds essential for crop growth, directly impacting food production.


• Construction: Cement, concrete, and ceramics are primarily composed of inorganic compounds, vital for building infrastructure.


• Electronics: Semiconductors (e.g., silicon, germanium), various metal oxides, and advanced ceramics are inorganic materials crucial for modern electronic devices.


• Metallurgy: The extraction and refining of metals often involve the preparation and use of various inorganic compounds.

2. Mohr's Salt (Ferrous Ammonium Sulfate, (NH₄)₂Fe(SO₄)₂·6H₂O)

Mohr's salt, a double salt of ferrous sulfate and ammonium sulfate, is a common and stable inorganic compound with significant applications, primarily in analytical chemistry:

• Redox Titrations: It is widely used as a primary standard in volumetric analysis, particularly in redox titrations involving oxidizing agents like potassium permanganate (KMnO₄) or potassium dichromate (K₂Cr₂O₇). Its stability in air, unlike FeSO₄, makes it ideal for accurate standardization.


• Iron Content Determination: In industrial settings, Mohr's salt solutions are used to calibrate instruments or to determine the concentration of oxidizing agents, which in turn can be used to analyze the iron content in various samples (e.g., ores, water).


• Educational Laboratories: Its ease of preparation and stability make it an excellent reagent for teaching fundamental principles of quantitative analysis.

3. Potash Alum (Potassium Aluminum Sulfate, KAl(SO₄)₂·12H₂O)

Potash alum, another double salt, exhibits a range of applications due to its chemical properties, especially its ability to form flocs and act as a mordant:

• Water Purification (Coagulant): This is one of its most important applications. In water treatment plants, potash alum is added to impure water to coagulate suspended impurities and colloidal particles. It forms a gelatinous precipitate of aluminum hydroxide, Al(OH)₃, which traps the fine particles, allowing them to settle down and be filtered out.


• Leather Tanning: Alum is used in the tanning process to harden leather and make it more resistant to decay. It works by cross-linking protein fibers in the animal hide.


• Dyeing Industry (Mordant): As a mordant, potash alum helps in fixing dyes to fabrics like cotton and wool. It forms an insoluble complex with the dye and the fabric fibers, preventing the dye from washing out easily.


• Paper Industry: It is used as a sizing agent, which helps in increasing the strength and decreasing the porosity of paper, preventing ink from spreading excessively.


• Medicine and Personal Care:
  
  
  
  • Styptic: Due to its astringent properties, it is used in aftershave lotions and styptic pencils to stop bleeding from minor cuts by constricting blood vessels and coagulating blood proteins.
  
  
  • Antiseptic/Antiperspirant: Its mild antiseptic properties make it useful in some deodorants and antiperspirants.
  
  
  
  


• Fire Retardants: Alum is sometimes incorporated into textiles or paper products to improve their flame resistance.

JEE & CBSE Relevance: Both Mohr's salt and potash alum are excellent examples of double salts often discussed in the context of coordination chemistry and practical inorganic chemistry. Their applications, especially in titrations (Mohr's salt) and water purification (potash alum), are frequently tested in objective and descriptive questions.

To effectively understand the chemistry involved in the preparation of inorganic compounds like Mohr's salt and potash alum, a strong foundation in several fundamental chemistry concepts is essential. These prerequisites ensure that you grasp the underlying principles and practical aspects of these preparations, which are crucial for both board exams and competitive exams like JEE.

Here are the key prerequisite concepts:

• Basic Stoichiometry and Mole Concept:
  
  
  
  • Understanding moles, molar masses, and how to perform calculations involving reactant and product quantities is fundamental.
  
  
  • Concepts like limiting reagent and theoretical yield are vital for predicting and optimizing the amount of product obtained in any synthesis.
  
  
  • JEE Relevance: Stoichiometric calculations often form part of questions related to synthesis and yield.
  
  
  
  


• Acid-Base Chemistry:
  
  
  
  • Knowledge of strong and weak acids/bases, pH, and neutralization reactions is important. Many inorganic preparations occur in specific pH ranges or involve acid/base additions to control reactions or purify products.
  
  
  • For example, acids might be used to dissolve starting materials or prevent hydrolysis.
  
  
  
  


• Solubility Rules and Precipitation:
  
  
  
  • Familiarity with common solubility rules for ionic compounds is crucial for understanding when a precipitate will form or when a compound will dissolve.
  
  
  • This is particularly relevant for separating desired products from impurities or for synthesizing compounds via precipitation reactions.
  
  
  
  


• Crystallization and Recrystallization:
  
  
  
  • Understanding the process of forming crystals from a saturated solution and the principles of recrystallization for purification is directly applicable to the preparation of both Mohr's salt and potash alum, which are isolated as crystals.
  
  
  • Factors affecting crystal size and purity are important considerations.
  
  
  
  


• Redox Reactions (Basic):
  
  
  
  • A basic understanding of oxidation states, oxidizing agents, and reducing agents is beneficial. While Mohr's salt preparation typically involves ferrous ions (Fe²⁺), if starting from ferric ions (Fe³⁺), a reduction step would be necessary.
  
  
  • Understanding disproportionation and combination reactions can also be relevant for other inorganic preparations.
  
  
  
  


• Chemical Bonding and Water of Crystallization:
  
  
  
  • Knowledge of ionic and covalent bonding helps in understanding the structure and stability of inorganic compounds.
  
  
  • The concept of 'water of crystallization' is directly applicable to both Mohr's salt (FeSO₄.(NH₄)₂SO₄.6H₂O) and potash alum (KAl(SO₄)₂.12H₂O), explaining their hydrated forms.
  
  
  
  


• Nomenclature of Inorganic Compounds:
  
  
  
  • Being able to correctly name and write chemical formulas for common inorganic compounds is a basic skill required to identify and understand the substances involved.
  
  
  
  

Mastering these foundational concepts will not only help you in understanding the specific preparations of Mohr's salt and potash alum but also build a strong base for tackling more complex inorganic chemistry topics. Approach these topics with clarity, as they are often interconnected.

The preparation of inorganic compounds like Mohr's salt and potash alum draws upon fundamental principles from various branches of chemistry. A strong grasp of these related topics is crucial for understanding the 'why' behind each step of the preparation process and for excelling in JEE Main examinations.

• 
  Stoichiometry and Mole Concept:
  
  
  
  • Relevance: This is foundational for calculating the exact quantities of reactants required to achieve the desired product yield. Precise mole ratios are essential for efficient synthesis and preventing the formation of by-products.
  
  
  • JEE Focus: Expect problems involving limiting reagents, theoretical yield, and percentage yield based on experimental data. This is equally critical for CBSE board exams.
  
  
  
  


• 
  Ionic Equilibrium:
  
  
  
  • Relevance: Concepts like solubility product (Ksp), common ion effect, and pH control are vital for understanding crystallization, dissolution, and precipitation processes involved in the purification of these salts. For instance, maintaining an acidic medium during Mohr's salt preparation prevents the oxidation of Fe(II) and the precipitation of ferric hydroxide.
  
  
  • JEE Focus: Deeper understanding of factors affecting solubility, buffer solutions, and the effect of pH on the precipitation of metal hydroxides is often tested.
  
  
  
  


• 
  Chemical Bonding and Structure:
  
  
  
  • Relevance: Understanding the nature of ionic bonds, hydration, and the difference between double salts (like Mohr's salt and potash alum, which dissociate completely in solution into their constituent ions) and coordination compounds (which retain their identity in solution) is crucial for characterizing the prepared compounds and predicting their behavior.
  
  
  • JEE Focus: Differentiating between various types of salts and their solution behavior is important.
  
  
  
  


• 
  Solid State Chemistry:
  
  
  
  • Relevance: The principles of crystallization, crystal lattice formation, and factors influencing crystal growth (e.g., slow cooling, presence of impurities) are fundamental to obtaining pure, well-formed crystals of the desired inorganic compound.
  
  
  • JEE Focus: While direct questions on crystallization mechanisms might be less common, understanding crystal defects and packing efficiency can provide a broader context.
  
  
  
  


• 
  Redox Reactions:
  
  
  
  • Relevance: While the direct preparation of Mohr's salt and potash alum are not typically redox reactions, understanding oxidation states and redox principles is important for handling precursors (e.g., ensuring Fe(II) remains as Fe(II) in Mohr's salt preparation) and appreciating the subsequent uses of these compounds (e.g., Mohr's salt as a primary standard in permanganometric titrations).
  
  
  • JEE Focus: Balancing redox reactions, identifying oxidizing/reducing agents, and electrochemistry concepts are frequently tested.
  
  
  
  


• 
  Qualitative and Quantitative Analysis of Inorganic Salts:
  
  
  
  • Relevance: Knowledge of identifying constituent ions (e.g., Fe²⁺, Al³⁺, K⁺, NH₄⁺, SO₄²⁻) in the prepared salts is essential for confirming their identity and purity. Quantitative analysis (e.g., titrations) can be used to determine the concentration or purity of the prepared compounds.
  
  
  • JEE Focus: Questions often combine analytical chemistry with preparation aspects, especially in practical chemistry sections.
  
  
  
  

Welcome to the "Common Exam Traps" section! Here, we'll highlight typical pitfalls and tricky areas encountered when studying the preparation of inorganic compounds like Mohr's salt and potash alum. Mastering these can significantly improve your exam performance.

Common Exam Traps & Mistakes

Students often lose marks not due to lack of knowledge, but by overlooking subtle details or falling for common misconceptions. Be vigilant about the following:

• Incorrect Chemical Formulas & Nomenclature:
  
  
  
  • Mohr's Salt: Many forget its hydrated form. It's FeSO₄.(NH₄)₂SO₄.6H₂O (Ferrous ammonium sulfate hexahydrate). Simply writing FeSO₄.(NH₄)₂SO₄ is incorrect for its common crystalline form.
  
  
  • Potash Alum: Similarly, it's K₂SO₄.Al₂(SO₄)₃.24H₂O (Potassium aluminum sulfate dodecahydrate). Misremembering the number of water molecules or the presence of both sulfates is a common error.
  
  
  • Trap: Being asked for the molar mass without specifying anhydrous vs. hydrated form – always assume the hydrated form unless stated otherwise for these specific compounds.
  
  
  
  


• Role of Specific Reagents/Conditions:
  
  
  
  • Mohr's Salt:
    
    
    
    • Dilute H₂SO₄: This is crucial. It's added to prevent the hydrolysis of ferrous sulfate (FeSO₄) and, more importantly, to inhibit the oxidation of Fe²⁺ to Fe³⁺ by atmospheric oxygen, which occurs more readily in neutral or alkaline conditions. A common trap is asking why FeSO₄ is not used alone or why acid is necessary.
    
    
    • Slow Cooling: Essential for the formation of large, well-defined crystals. Rapid cooling leads to small, impure crystals.
    
    
    
    
  
  
  • Potash Alum:
    
    
    
    • Solubility: Understanding the solubility difference between potassium sulfate and aluminum sulfate at different temperatures is key to successful crystallization.
    
    
    • Acidity: The solution of potash alum is acidic due to the hydrolysis of Al³⁺ ions. Don't confuse this with strong acid properties; it's due to the hydrolysis of the salt.
    
    
    
    
  
  
  
  


• Distinguishing Double Salts from Complex Salts:
  
  
  
  • Both Mohr's salt and Potash Alum are double salts. This means they dissociate completely into their constituent ions in aqueous solution (e.g., Fe²⁺, NH₄⁺, SO₄^2- for Mohr's salt).
  
  
  • Trap: Confusing them with complex salts (coordination compounds) which retain their complex ion structure in solution (e.g., [Fe(CN)₆]^4- from K₄[Fe(CN)₆]). This is a fundamental distinction tested frequently.
  
  
  • JEE Focus: Questions often test this conceptual understanding, asking about the ions present in solution or differentiating between double and complex salts.
  
  
  
  


• Practical Yield & Purity Calculations:
  
  
  
  • While direct preparation steps are more common in CBSE, JEE might include questions on theoretical yield, percentage yield, or purity calculations based on the amounts of starting materials.
  
  
  • Mistake: Not considering the limiting reagent or impurities in starting materials.
  
  
  
  


• Reaction Stoichiometry:
  
  
  
  • Ensuring the correct molar ratios of reactants (e.g., ferrous sulfate and ammonium sulfate for Mohr's salt, or potassium sulfate and aluminum sulfate for potash alum) is critical.
  
  
  • Trap: Questions might provide unequal molar amounts and ask about the limiting reactant or theoretical yield.
  
  
  
  

By understanding these common traps, you can approach questions on the preparation of Mohr's salt and potash alum with greater confidence and accuracy. Good luck!

These key takeaways summarize the essential chemical principles and practical considerations for the preparation of specific inorganic compounds like Mohr's salt and potash alum, crucial for both theoretical understanding and laboratory-based questions in competitive exams.

Key Takeaways: Chemistry Involved in Preparation

The preparation of inorganic compounds often involves principles of solubility, crystallization, and maintaining specific chemical environments to ensure purity and stability. Let's focus on Mohr's Salt and Potash Alum.

1. Mohr's Salt (Ferrous Ammonium Sulfate, FAS)

• Nature: Mohr's salt, with the formula FeSO₄.(NH₄)₂SO₄.6H₂O, is a double salt. It crystallizes as a single substance but dissociates completely into its constituent ions (Fe²⁺, NH₄⁺, SO₄^2-) in aqueous solution.


• Reactants: It is prepared by the crystallization of an equimolar mixture of ferrous sulfate (FeSO₄) and ammonium sulfate ((NH₄)₂SO₄) from an acidic aqueous solution.


• Role of Sulfuric Acid: A small amount of dilute sulfuric acid is added during preparation.
  
  
  
  • JEE/CBSE Focus: This is crucial. The acidic medium prevents the hydrolysis of Fe²⁺ ions (which would form Fe(OH)₂) and, more importantly, inhibits the oxidation of Fe²⁺ to Fe³⁺ by atmospheric oxygen. Fe³⁺ would lead to a brown precipitate.
  
  
  
  


• Advantage over Ferrous Sulfate: Mohr's salt is preferred as a primary standard in titrations over ferrous sulfate because it is more stable against aerial oxidation and does not effloresce (lose water of crystallization) readily.


• Crystallization: The solution is heated to dissolve reactants, filtered if necessary, and then allowed to cool slowly to obtain well-formed, pale green crystals. Slow cooling promotes larger, purer crystals.

2. Potash Alum (Potassium Aluminium Sulfate)

• Nature: Potash alum, with the formula K₂SO₄.Al₂(SO₄)₃.24H₂O, is also a double salt and belongs to the general class of 'alums'. It crystallizes as a single substance and dissociates into K⁺, Al³⁺, and SO₄^2- ions in water.


• Reactants: Typically prepared from an equimolar solution of potassium sulfate (K₂SO₄) and aluminium sulfate (Al₂(SO₄)₃). Aluminium sulfate itself can be prepared by dissolving aluminium metal, aluminium oxide (Al₂O₃) or aluminium hydroxide (Al(OH)₃) in sulfuric acid.


• Preparation Steps (common method from waste aluminium):
  
  
  
  1. Waste aluminium is reacted with concentrated potassium hydroxide solution to form potassium meta-aluminate: 2Al + 2KOH + 2H₂O → 2KAlO₂ + 3H₂
  
  
  2. The potassium meta-aluminate solution is then treated with sulfuric acid, which precipitates aluminium hydroxide: 2KAlO₂ + H₂SO₄ + 2H₂O → 2Al(OH)₃ + K₂SO₄
  
  
  3. The precipitated Al(OH)₃ is dissolved in more sulfuric acid to form aluminium sulfate: 2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O
  
  
  4. Finally, the aluminium sulfate solution is mixed with potassium sulfate solution (or K₂SO₄ formed in step 2) and allowed to crystallize: K₂SO₄ + Al₂(SO₄)₃ + 24H₂O → K₂SO₄.Al₂(SO₄)₃.24H₂O
  
  
  
  


• Crystallization: Similar to Mohr's salt, slow cooling of a hot, saturated solution leads to the formation of large, octahedral crystals of potash alum.


• JEE/CBSE Focus: Understanding the steps involved, particularly how aluminium compounds (like Al(OH)₃) are converted into soluble aluminium sulfate, and the role of crystallization in obtaining the double salt.

🧩 Problem Solving Approach

A systematic approach is crucial for tackling problems related to the preparation of inorganic compounds like Mohr's salt and potash alum in competitive exams.

I. Core Principles for Preparation Problems:

• Balanced Chemical Equations & Stoichiometry:
  
  
  
  • Always begin by identifying the reactants and products and writing the balanced chemical equation(s) involved in the preparation.
  
  
  • Problems often test your ability to calculate theoretical yield, limiting reagents, or percentage yield. Ensure you are comfortable with mole concepts and mass-mass relationships.
  
  
  • JEE Focus: Stoichiometric calculations are very common. Be precise with molecular weights and mole ratios.
  
  
  
  


• Role of Reaction Conditions:
  
  
  
  • Understand why specific conditions (temperature, pH, concentration) are used. For example, heating/cooling rates for crystallization, or acidic/basic media to prevent side reactions.
  
  
  • Questions might ask about the consequence of deviating from these conditions.
  
  
  
  


• Purity and Crystallization Techniques:
  
  
  
  • Many preparations involve crystallization as a purification step. Know the principles of solubility, supersaturation, and factors affecting crystal size and purity (e.g., slow cooling yields larger, purer crystals).
  
  
  • Recrystallization is often employed to enhance purity.
  
  
  
  

II. Specific Considerations for Mohr's Salt (Ferrous Ammonium Sulfate, FeSO₄·(NH₄)₂SO₄·6H₂O):

• Prevention of Oxidation:
  
  
  
  • Key Problem: Fe²⁺ ions tend to oxidize to Fe³⁺ (especially in neutral or alkaline solutions).
  
  
  • Solution: The preparation is carried out in an acidic medium (dilute H₂SO₄). This suppresses the oxidation of Fe²⁺ and prevents hydrolysis of Fe²⁺ salts.
  
  
  • Understand why concentrated sulfuric acid is avoided (risk of dehydration or side reactions).
  
  
  
  


• Double Salt Nature:
  
  
  
  • Mohr's salt is a double salt, meaning it dissociates completely into its constituent ions (Fe²⁺, NH₄⁺, SO₄²⁻) when dissolved in water. This is a critical distinction from complex salts.
  
  
  • Questions might compare its behavior with that of complex compounds.
  
  
  
  

III. Specific Considerations for Potash Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O):

• Source of Aluminium:
  
  
  
  • Preparation usually starts from aluminium scrap (metal) or aluminium oxide (Al₂O₃).
  
  
  • Step 1 (Aluminate Formation): Aluminium metal reacts with a strong base (like KOH or NaOH) to form potassium aluminate (K[Al(OH)₄]) or sodium aluminate.
    
    2Al + 2KOH + 6H₂O → 2K[Al(OH)₄] + 3H₂
    
  
  
  • Step 2 (Aluminum Hydroxide Precipitation): The aluminate solution is then acidified with dilute H₂SO₄ to precipitate aluminium hydroxide (Al(OH)₃).
    
    K[Al(OH)₄] + H₂SO₄ → Al(OH)₃(s) + KHSO₄ + H₂O
    
  
  
  • Step 3 (Alum Formation): The Al(OH)₃ precipitate is dissolved in more dilute H₂SO₄ to form aluminium sulfate. This solution is then mixed with a solution of potassium sulfate (K₂SO₄) and subjected to crystallization.
    
    2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O

Al₂(SO₄)₃ + K₂SO₄ + 24H₂O → K₂SO₄·Al₂(SO₄)₃·24H₂O (Potash Alum)
    
  
  
  
  


• Alum Nature:
  
  
  
  • Potash alum is a type of double salt belonging to the alum family (M₂SO₄·M'₂(SO₄)₃·24H₂O, where M is a monovalent cation and M' is a trivalent cation).
  
  
  • Its high hydration (24 water molecules) is characteristic.
  
  
  
  

IV. General Problem-Solving Steps:

1. Read Carefully: Understand what is asked – e.g., product quantity, reason for a specific step, conditions required, or a comparison.


2. Recall Reactions: Write down the balanced chemical equations pertinent to the problem.


3. Identify Key Principles: Determine which chemical concepts are being tested (e.g., redox, acid-base, solubility, stoichiometry).


4. Apply Logic: Use the recalled principles and equations to derive the solution. For "reasoning" questions, focus on the underlying chemistry.


5. Check Units & Significant Figures: Especially for numerical problems in JEE.

JEE Tip: Questions often combine preparation methods with properties or uses. Be prepared to link these concepts.