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

Step-by-Step Solution

Answer

The pseudo alum is (d) \( \mathrm{NH}_4 \mathrm{Fe} \left(\mathrm{SO}_4\right)_{2} \cdot 12 \mathrm{H}_2 \mathrm{O}\).

Step 1: Understanding Alums

Alums are a type of double sulfate compounds, typically with the general formula \[ \text{X}_2\text{SO}_4 \cdot \text{Y}_2(\text{SO}_4)_3 \cdot 24\text{H}_2\text{O} \]where \(\text{X}\) is a monovalent cation (such as \(\text{K}^+\) or \(\text{NH}_4^+\)) and \(\text{Y}\) is a trivalent cation (such as \(\text{Al}^{3+}\) or \(\text{Cr}^{3+}\)) associated with aluminum. Pseudo alums are similar to alums but have a different trivalent metal ion.

Step 2: Definition and Identification of Pseudo Alums

Pseudo alums have a similar composition to standard alums but contain different trivalent metal ions such as iron or chromium instead of aluminum. The chemical structure still involves double sulfates and water of crystallization.

Step 3: Analyzing the Options

We have to identify the compound which fits the definition of a pseudo alum: - Option (a): Contains aluminum, which is a standard alum. - Option (b): Contains chromium, where the standard alum would contain aluminum. - Option (c): Contains manganese, which does not typically form a common alum or pseudo alum. - Option (d): Contains iron instead of aluminum, fitting the description of pseudo alum.

Step 4: Determining the Correct Answer

Comparing the provided options with what constitutes a pseudo alum, option (d) \( \mathrm{NH}_4 \mathrm{Fe} \left(\mathrm{SO}_4\right)_{2} \cdot 12 \mathrm{H}_2 \mathrm{O} \)fits the description as it contains iron instead of aluminum.

Key Concepts

Double Sulfate Compounds

Double sulfate compounds are fascinating chemistry creations. They are formed when two different sulfate ions combine in a specific ratio. These compounds have a general structure, where one type of cation is monovalent and another is trivalent.
For example:

• A common formula is \( ext{X}_2 ext{SO}_4 \cdot ext{Y}_2( ext{SO}_4)_3 \cdot 24 ext{H}_2 ext{O}\), where \( ext{X}\) usually represents a single-positive charged ion like potassium \( ext{K}^+\).
• The \( ext{Y}\) represents a triply positive ion like aluminum \( ext{Al}^{3+}\).

This arrangement forms a crystalline structure with water molecules fitting in specific spots, creating what we call 'hydrated' minerals. The water of crystallization plays a crucial role in stabilizing these compounds.
Therefore, double sulfate compounds possess an aesthetic nature owing to their crystalline forms and the intersection of two different types of cations, leading to a unique synergy across the atomic arrangement.

Trivalent Metal Ions

In the realm of chemistry, trivalent metal ions hold a special place. These ions have lost three electrons, giving them a 3+ charge.
You’ll frequently find trivalent metal ions like:

• Aluminum \(\text{Al}^{3+}\)
• Chromium \(\text{Cr}^{3+}\)
• Iron \(\text{Fe}^{3+}\)

These ions play critical roles in forming double sulfate compounds. They influence the electric neutrality and the structural integrity of the molecule. By holding a +3 charge, trivalent metal ions act as central points attracting the negatively charged sulfate ions.
Each trivalent ion brings unique properties, leading to different variants of alum: real alum with aluminum and pseudo alum with other trivalent ions like iron or chromium. These differences can adjust the physical and chemical properties of the resulting crystal, greatly affecting their applications and stability.

Alum Chemistry

Alum chemistry focuses on an intriguing group of compounds known as alums and pseudo alums. Alums are usually recognized by their common formula \(\text{X}_2\text{SO}_4 \cdot \text{Y}_2(\text{SO}_4)_3 \cdot 24\text{H}_2\text{O}\).
Alums are double sulfate compounds that have both a monovalent and a trivalent metal ion. Here are some key characteristics:

• They are structurally bound with water molecules, giving them a unique form of crystalline hydrate.
• Pseudo alums are similar to real alums but contain different trivalent ions like iron or chromium.

Identifying a pseudo alum requires recognizing the variance in trivalent metal ions within the compound's structure. For example, \(\text{NH}_4 \text{Fe}(\text{SO}_4)_2 \cdot 12 \text{H}_2 \text{O}\) is a classic representation of a pseudo alum because iron replaces the usual aluminum in its framework.
Therefore, the subtle differences in metal ions create diverse alum types, which alter their physical properties and applications, from being used in water purification to medicinal purposes. Understanding alum chemistry not only involves knowing the elements involved but also how these metals influence each compound’s characteristics and uses.