When ${\mathrm{H}}_2\mathrm{S}$ gas is passed into a mixture of ${\mathrm{Mn}}^{2+}, {\mathrm{Ni}}^{2+}, {\mathrm{Cu}}^{2+}$ and ${\mathrm{Hg}}^{2+}$ ion in an acidified aqueous solution, the precipitates formed are

In presence of acidic medium, ionisation of ${\mathrm{H}}_2\mathrm{S}$ is supressed so less number of ${\mathrm{S}}^{2-}$ ions are produced. So only those sulphides are precipitated which have low solubility product $\left({\mathrm{K}}_{\mathrm{sp}}\right)$ value.

Thus, in acidic medium, ${\mathrm{H}}_2\mathrm{S}$ is very feebly ionised giving very small concentration of sulphide ion for precipitation. Therefore, the most insoluble salts $\mathrm{CuS}$ and $\mathrm{HgS}$ are precipitated.