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  4. ZnS is not precipitated by passing H2S in acidic medium but CuS is precipitated. The reason for it is

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Q. $ZnS$ is not precipitated by passing $H_2S$ in acidic medium but $CuS$ is precipitated. The reason for it is

Equilibrium

Solution:

$H_{2}S \rightleftharpoons 2H^{+}+S^{2-}$
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