Question

$ZnS$ is not precipitated by passing $H_{2}S$ in acidic medium but $CuS$ is precipitated. The reason for it is

• A. $K_{sp}CuS<<K_{sp}ZnS$
• B. $K_{sp}CuS>>K_{sp}ZnS$
• C. $K_{sp}CuS=K_{sp}ZnS$
• D. None of these.

Answer 1

Answer: (A)

$H_{2}S\rightleftharpoons 2H^{+}+S^{2-}$
In acidic medium ionisation of $H_{2}S$ is suppressed due to common ion effect. This decreases the concentration of $S^{2-}$ ions in solution. As such $K_{sp}$ of only $CuS$ is exceeded resulting in the precipitation of $CuS$. $K_{sp}$ of $ZnS$ is much higher