AIIMS 2005 Chemistry Solubility Assertion Reason Question

To address the given question, we need to analyze both the assertion and the reason carefully.

Assertion Analysis

The assertion states: "Addition of NH₄OH to an aqueous solution of BaCl₂ in the presence of NH₄Cl (excess) precipitates Ba(OH)₂."

1. Understanding the Components:

   • BaCl₂ is barium chloride, which dissociates in water to give Ba²⁺ ions and Cl⁻ ions: $\text{BaCl}_2 \rightarrow \text{Ba}^{2+} + 2 \text{Cl}^-$
   • NH₄OH is ammonium hydroxide, which can also be represented as NH₄⁺ and OH⁻ in solution.
   • When NH₄OH is added to the Ba²⁺ solution, it introduces OH⁻ ions, which can react with Ba²⁺ to form Ba(OH)₂: $\text{Ba}^{2+} + 2 \text{OH}^- \rightarrow \text{Ba(OH)}_2$

2. The Role of NH₄Cl:

   • NH₄Cl provides an excess of NH₄⁺ ions, which can suppress the solubility of Ba(OH)₂ through the common ion effect. However, Ba(OH)₂ is not significantly soluble in water, leading to precipitation.

3. Conclusion for Assertion:

   • The assertion is false because while Ba(OH)₂ can precipitate from Ba²⁺ and OH⁻, the presence of excess NH₄Cl will actually keep Ba(OH)₂ in solution due to the common ion effect, preventing its precipitation.

Reason Analysis

The reason states: "Ba(OH)₂ is insoluble in water."

1. Solubility of Ba(OH)₂:

   • Barium hydroxide, Ba(OH)₂, is indeed considered to be sparingly soluble in water. The solubility product (Ksp) for Ba(OH)₂ at room temperature is quite low, indicating that it does not dissolve well in water.

2. Conclusion for Reason:

   • The reason is true; however, it does not explain the assertion correctly.

Answer Explanation

Based on the analysis above:

• The assertion is false because the presence of NH₄Cl prevents Ba(OH)₂ from precipitating due to the common ion effect.
• The reason is true but does not adequately explain why the assertion is incorrect.

Thus, the correct answer is D) If both assertion and reason are false.

Clarification of Other Options

• Option A: If both assertion and reason are true and reason is the correct explanation of assertion - Incorrect because the assertion is false.
• Option B: If both assertion and reason are true but reason is not the correct explanation of assertion - Incorrect because the assertion is false.
• Option C: If assertion is true but reason is false - Incorrect because the assertion is false.

Summary

In summary, the assertion regarding the precipitation of Ba(OH)₂ in the presence of excess NH₄Cl is false due to the common ion effect, while the reason is true but does not support the assertion. Therefore, option D is the correct answer.