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) \(\mathrm{KCr}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\) (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

(d) \(\mathrm{NH}_{4} \mathrm{Fe}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\) is the pseudo alum.

Step 1: Understand the Question

The question is asking us to identify the pseudo alum from the given options. A pseudo alum is a double sulfate that differs from a true alum by having an ammonium or a different trivalent metal instead of aluminum.

Step 2: Identify the Components of a True Alum

A true alum has the general formula \(\mathrm{XY\left(SO_{4}\right)_{2} \cdot 12H_{2}O}\), where \(\mathrm{X}\) is typically potassium, sodium, or ammonium, and \(\mathrm{Y}\) is aluminum with a charge of 3+.

Step 3: Analyze Each Option

Look for the presence of components that replace aluminum with a different trivalent metal or use ammonium:- (a) \(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}\): has aluminum but not a double sulfate form.- (b) \(\mathrm{KCr}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\): chromium replaces aluminum, fits the pseudo alum pattern.- (c) \(\mathrm{KMn}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\): manganese present instead of aluminum.- (d) \(\mathrm{NH}_{4} \mathrm{Fe}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\): ammonium and iron present.

Step 4: Select the Pseudo Alum

Based on the analysis, the compounds (b), (c), and (d) all could be pseudo alums. However, the classic sense of pseudo alum often refers to ammonium ions replacing potassium with a trivalent metal other than aluminum. Thus, the correct answer is (d) \(\mathrm{NH}_{4} \mathrm{Fe}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\).

Key Concepts

Double Sulfate

Double sulfates are quite interesting in the world of chemistry. They consist of two different kinds of metal ions combined with the sulfate ion ( \(\mathrm{SO}_4^{2-}\)). These substances form crystalline structures. They are often highly soluble in water and display distinctive colors. For students, knowing these characteristics can help in identifying compounds in experiments or exams. In the context of pseudo alums, these compounds maintain the sulfate group's key role but include different metal ions, creating a fascinating blend of properties. Additionally, double sulfates form as a result of specific crystallization processes, which make them unique. They are essential in various industrial applications due to their notable stability and ionic properties.

Trivalent Metal

The term "trivalent metal" refers to metal ions that have a charge of +3. Examples include aluminum (\(\mathrm{Al}^{3+}\)), chromium (\(\mathrm{Cr}^{3+}\)), and iron (\(\mathrm{Fe}^{3+}\)). These metals are key components in the formation of alums and pseudo alums.In true alums, aluminum is the most common trivalent metal. However, in pseudo alums, other trivalent metals replace aluminum. This substitution modifies the compound's properties, making them distinct. Understanding the nature of trivalent metals is crucial for grasping why these substances behave differently in chemical reactions. Their ability to form intricate networks by bonding with sulfate groups is a fundamental aspect of these compounds.

Ammonium Replacement

In the context of pseudo alums, ammonium ions (\(\mathrm{NH_4^+}\)) frequently replace potassium ions (\(\mathrm{K^+}\)) found in true alums. Ammonium's versatility and ability to establish similar crystalline networks make it a favorite in this exchange. This change influences the overall characteristics of the compound, including its solubility and crystallization behavior. When ammonium replaces other ions, typically alongside a trivalent metal, we often observe altered chemical properties. This swapping is the foundation for creating pseudo alums. Hence, it's important to recognize this concept, as it helps in predicting both the potential uses and reactions of the compound.

True Alum

True alums are classic examples of double salts, combining trivalent metals with sulfate ions and a univalent metal such as potassium or sodium. The basic formula for a true alum is \(\mathrm{XY\left(SO_4\right)_2 \cdot 12H_2O}\), where "X" can be a univalent metal and "Y" is a trivalent metal, usually aluminum.True alums are known for their historical and practical uses. They are used in everything from water purification to pickling processes. The unique balance of metals and sulfate ions grants them fascinating properties such as forming into beautiful octahedral crystals.Understanding how true alums are structured helps in recognizing how pseudo alums differ from them. These differences include changes in the metal ions present, which lead to varied chemical behaviors.

Aluminum Sulfate

Aluminum sulfate (\(\mathrm{Al_2(SO_4)_3}\)) is a vital component in many true alums. It acts as the trivalent metal that bonds with sulfate ions to create the distinct alum structure.This compound is known for its role in water treatment. It helps in removing impurities through a process called flocculation, where particles clump together for easy removal.In addition, aluminum sulfate is key in the development of dyes and papers. As part of alums, its presence determines the solubility and crystallization characteristics of the compound. Understanding the significance of aluminum sulfate allows us to better understand how its replacement with other trivalent metals leads to the formation of pseudo alums.