Chemical and Ionic Equilibrium 2 Question 18
18. Identify the correct order of solubility of $\mathrm{Na}_{2} \mathrm{~S}, \mathrm{CuS}$ and $\mathrm{ZnS}$ in aqueous medium.
(2002)
(a) $\mathrm{CuS}>\mathrm{ZnS}>\mathrm{Na}_{2} \mathrm{~S}$
(b) $\mathrm{ZnS}>\mathrm{Na}_{2} \mathrm{~S}>\mathrm{CuS}$
(c) $\mathrm{Na}_{2} \mathrm{~S}>\mathrm{CuS}>\mathrm{ZnS}$
(d) $\mathrm{Na}_{2} \mathrm{~S}>\mathrm{ZnS}>\mathrm{CuS}$
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Solution:
- Alkali metal salts are usually more soluble than the salts of transition metals. Also, $\mathrm{CuS}$ is less soluble than $\mathrm{ZnS}$ because of $3 d^{9}$ configuration of $\mathrm{Cu}^{2+}$. Therefore, solubility order is
$$ \mathrm{Na}_{2} \mathrm{~S}>\mathrm{ZnS}>\mathrm{CuS} $$
