When H _ 2 S gas is passed into a mixture of Mn ^ 2 + , Ni ^ 2 + ,...

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

Thus, in acidic medium, ${H}_{2}S$ is very feebly ionised giving very small concentration of sulphide ion for precipitation. Therefore, the most insoluble salts $CuS and HgS$ are precipitated.