Why only $\mathrm{As}^{3+}$ gets precipitated as $\mathrm{As}_{2} \mathrm{~S}_{3}$ and not…
Why only $\mathrm{As}^{3+}$ gets precipitated as $\mathrm{As}_{2} \mathrm{~S}_{3}$ and not $\mathrm{Zn}^{2+}$ as $\mathrm{ZnS}$ when $\mathrm{H}_{2} \mathrm{~S}$ is passed through an acidic solution containing $\mathrm{As}^{3+}$ and $\mathrm{Zn}^{2+}$ ?
Solubility product of $\mathrm{As}_{2} \mathrm{~S}_{3}$ is less than that of $\mathrm{ZnS}$
Enough $\mathrm{As}^{3+}$ are present in acidic medium
Zinc salt does not ionise in acidic medium
Solubility product changes in presence of an acid
Solution
$\mathrm{K}_{\mathrm{sp}}$ of $\mathrm{As}_{2} \mathrm{~S}_{3}$ is less than $\mathrm{ZnS}$. In acidic medium ionisation of $\mathrm{H}_{2} \mathrm{~S}$ is suppresed (common ion effect) and $\mathrm{K}_{\mathrm{sp}}$ of $\mathrm{ZnS}$ does not exceed.
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