AIIMS 2006 Chemistry Solubility Assertion Reason Question

To analyze the given question, we will first examine the assertion and the reason provided.

Assertion:

Sb₂S₃ is not soluble in yellow ammonium sulphide.

Reason:

The common ion effect due to S²⁻ ions reduces the solubility of Sb₂S₃.

Analysis:

1. Understanding the Assertion:

   • Antimony trisulfide, $\text{Sb}_2\text{S}_3$, is indeed poorly soluble in many solutions.
   • Yellow ammonium sulfide, $\text{(NH}_4)_2\text{S}$, contains sulfide ions, $\text{S}^{2-}$, which are the same ions that make up $\text{Sb}_2\text{S}_3$.

2. Understanding the Reason:

   • The common ion effect states that the solubility of a salt is reduced in a solution that contains a common ion. For $\text{Sb}_2\text{S}_3$, the presence of $\text{S}^{2-}$ ions from yellow ammonium sulfide would indeed lead to a decrease in its solubility due to this effect.
   • However, the assertion that $\text{Sb}_2\text{S}_3$ is “not soluble” in yellow ammonium sulfide is misleading. It implies that it cannot dissolve at all, which is not accurate; it simply has reduced solubility.

3. Conclusion on the Correct Answer:

   • Therefore, while the assertion is misleading, it is ultimately false because $\text{Sb}_2\text{S}_3$ does have some solubility, albeit low. The reason, while true about the common ion effect reducing solubility, does not explain the assertion that it is "not soluble" as it overlooks the fact that the compound can still dissolve to an extent.

Correct Answer:

D) Both assertion and reason are false statements.

Why Other Options are Incorrect:

• Option A: Incorrect because both the assertion and reason are not true. $\text{Sb}_2\text{S}_3$ is not completely insoluble.
• Option B: Incorrect for the same reason; the assertion is not true, and thus the reason cannot be a correct explanation.
• Option C: Incorrect because it states that the assertion is true, while it is not.

Relevant Concepts:

• Common Ion Effect: This effect describes how the solubility of a salt decreases in a solution that contains a common ion.
• Solubility Product Constant ($K_{sp}$): The solubility of $\text{Sb}_2\text{S}_3$ can be described by its $K_{sp}$: $K_{sp} = [\text{Sb}^{3+}]^2[\text{S}^{2-}]^3$ The presence of $\text{S}^{2-}$ from yellow ammonium sulfide would shift the equilibrium, reducing the amount of $\text{Sb}^{3+}$ ions produced.

Summary:

In conclusion, the assertion that $\text{Sb}_2\text{S}_3$ is not soluble in yellow ammonium sulfide is false. The reason regarding the common ion effect is true, but it does not support the assertion correctly. Hence, the correct answer is D, as both statements are false.