Question

For the following Assertion and Reason, the correct option is

Assertion (A): When Cu (II) and sulphide ions are mixed, they react together extremely quickly to give a solid.

Reason (R): The equilibrium constant of \(\mathrm{Cu^{2+}(aq) + S^{2-}(aq) \rightleftharpoons CuS(s)}\) is high because the solubility product is low.

• A. (A) is false and (R) is true.
• B. Both (A) and (R) are true but (R) is not the explanation for (A).
• C. Both (A) and (R) are true and (R) is the explanation for (A).
• D. Both (A) and (R) are false.

Hint:

For precipitation, \(K = 1/K_{sp}\); a very small \(K_{sp}\) means a very large K and almost instantaneous formation of the solid.

Answer 1

The Correct Option is C

For \(\mathrm{Cu^{2+} + S^{2-} \rightleftharpoons CuS(s)}\), the equilibrium constant \(K = \dfrac{1}{K_{sp}}\).
CuS has an extremely small \(K_{sp}\) (about \(10^{-36}\)), so K is very large. Hence (R) is true.
Because the ionic product exceeds \(K_{sp}\) enormously, the ions combine essentially instantly to precipitate CuS (black solid). Hence (A) is true.
The very low solubility product (high K) is the reason for the immediate, complete precipitation, so (R) explains (A).
Answer: (3) Both (A) and (R) are true and (R) is the explanation for (A).