Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is known as pseudo alum? (a) \(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{KCr}\left(\mathrm{SO}_{4}\right)_{2}, 12 \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{KMn}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{NH}_{4} \mathrm{Fe}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Option (c) is known as pseudo alum.

Step 1: Understanding the Definition

A pseudo alum is a compound similar to a true alum but with different cationic or anionic components. Alums typically have the formula \ MIMIII\(\text{(SO}_4\text{)}_2\cdot n\text{H}_2\text{O}\), where MI is a monovalent cation, like K+, and MIII is a trivalent metal cation, often Al3+.

Step 2: Analyzing Each Option

- Option (a): Contains both K+ (monovalent) and Al3+ (trivalent) and fits the normal alum structure, so it's not a pseudo alum. - Option (b): Contains K+ and Cr3+; this fits the alum structure. - Option (c): Contains KMn(SO4)2 and appears to be an alum structure but with Mn3+, a substitute for the usual trivalent metal cations. - Option (d): Contains NH4+ and Fe3+, which also fits the structure of a pseudo alum since NH4+ is replacing a typical monovalent cation like K+.

Step 3: Identifying the Pseudo Alum

Review the options for elements replacing typical components in an alum. Option (c), KMn(SO4)2, 12H2O, uses Mn3+ instead of a usual trivalent cation like Al3+ or Fe3+, hence considered a pseudo alum.

Key Concepts

Pseudo alum

A pseudo alum is a fascinating type of chemical compound. The term refers to a compound that resembles a true alum but includes slight variations in its chemical components. True alums typically follow the formula \( M^I M^{III} ( \text{SO}_4 )_2 \cdot n \text{H}_2\text{O} \). Here, \( M^I \) is a monovalent cation, like \(\text{K}^+\), and \( M^{III} \) is a trivalent metal cation such as \(\text{Al}^{3+}\).

• A pseudo alum, however, has different cationic or anionic components.
• These variations give pseudo alums differing properties from true alums while maintaining a similar crystalline structure.

To illustrate this concept, consider \(\text{KMn(SO}_4\text{)}_2 \cdot 12\text{H}_2\text{O}\), a pseudo alum where \(\text{Mn}^{3+}\) replaces a typical trivalent cation like \(\text{Al}^{3+}\). This substitution changes some physical and chemical properties, classifying it as a pseudo alum.

Alum structure

The structure of alums is key to understanding both their formation and their forms as true and pseudo alums. Alums are typically known for their specific crystalline structure:

• Their general chemical formula is \( M^I M^{III} ( \text{SO}_4 )_2 \cdot n \text{H}_2\text{O} \).
• This structure consists of alternating layers of monovalent and trivalent metal cations with sulfate (\(\text{SO}_4^{2-}\)) anions and water molecules.

The symmetry and repetitive nature of this structure lead to well-formed octahedral crystals. Alums are versatile due to their phytostability and ability to incorporate different ions.The ability to substitute the monovalent or trivalent ions results in different types of alums, such as pseudo alums, where components like \(\text{NH}_4^+\) might replace \(\text{K}^+\), or other trivalent cations like \(\text{Mn}^{3+}\) might replace \(\text{Al}^{3+}\), providing these compounds with a variety of uses in different industries.

Trivalent metal cations

Trivalent metal cations are pivotal for the formation of alums. These cations have a \(+3\) charge and are crucial for maintaining the structural integrity of alum compounds. Common trivalent cations include:

• \( \text{Al}^{3+} \) - the standard in many alums.
• \( \text{Fe}^{3+} \) - often used in \(\text{NH}_4\text{Fe(SO}_4\text{)}_2 \cdot 12\text{H}_2\text{O}\), a type of pseudo alum.
• \( \text{Cr}^{3+} \) - found in some chromium-based alums.

These cations form robust ionic structures when combined with sulfate ions and water. Additionally, they help define the physical and chemical properties of alums, including aspects like solubility, coloration, and thermal stability.In pseudo alums, alternative trivalent cations, like \(\text{Mn}^{3+}\), can be introduced, altering the compound's characteristics while keeping the fundamental alum framework.