Question

Assertion: Addition of silver ions to a mixture of aqueous sodium chloride and sodium bromide solution will first precipitate AgBr rather than AgCl.

Reason: $K_{sp}$ of AgCl > $K_{sp}$ of AgBr .

• A. Assertion is correct, reason is correct; reason is a correct explanation for assertion.
• B. Assertion is correct, reason is correct; reason is not a correct explanation for assertion
• C. Assertion is correct, reason is incorrect
• D. Assertion is incorrect, reason is correct.

Answer 1

Answer: (A)

Assertion is correct, reason is correct; reason is a correct explanation for assertion.

Answer 2

When silver ions ($Ag^+$) are added to a solution containing chloride ($Cl^-$) and bromide ($Br^-$) ions, two sparingly soluble salts, AgCl and AgBr, can potentially precipitate. The precipitation of a salt occurs when its ionic product (Qsp) exceeds its solubility product ($K_{sp}$). For salts with the same stoichiometry (like AgCl and AgBr, both 1:1 electrolytes), the one with the lower $K_{sp}$ value will precipitate first, assuming initial concentrations of the anions are comparable. This is because a lower concentration of $Ag^+$ ions is required to reach the saturation point for the salt with the smaller $K_{sp}$.

Let's look at the solubility product values:

• $K_{sp}(AgCl) \approx 1.8 \times 10^{-10}$
• $K_{sp}(AgBr) \approx 5.0 \times 10^{-13}$

From these values, it is clear that $K_{sp}(AgBr) < K_{sp}(AgCl)$, or equivalently, $K_{sp}(AgCl) > K_{sp}(AgBr)$.

Assertion Analysis:

The assertion states that AgBr will precipitate first. Since $K_{sp}(AgBr)$ is significantly smaller than $K_{sp}(AgCl)$, AgBr is less soluble than AgCl. Therefore, a lower concentration of $Ag^+$ is needed to initiate the precipitation of AgBr compared to AgCl. Hence, AgBr will precipitate first. The assertion is correct.

Reason Analysis:

The reason states that $K_{sp}$ of AgCl > $K_{sp}$ of AgBr. As confirmed by the actual $K_{sp}$ values, this statement is correct.

Conclusion:

Both the assertion and the reason are correct. Furthermore, the reason (that AgCl has a higher $K_{sp}$ than AgBr, meaning AgBr has a lower $K_{sp}$) directly explains why AgBr precipitates first (because it is less soluble and requires a lower $Ag^+$ concentration to precipitate). Therefore, the reason is a correct explanation for the assertion.