Why are the group IV cations not precipitated as sulphides on passing `H_(2)S` gas through group II solution?

Explanation

For the precipitation of cations of group IV, a very large concentration of sulphide ions `(S^(2-))` id required which is not available in the group II as the ionisation of `H_(2)S` is suppressed by HCl (common ion effect). In precipitating the cations of group IV high concentration of `S^(2-)` ions is required as `OH^(ɵ)` ions released by `NH_(4)OH` unite with `H^(o+)` ions given by `H_(2)S`. Thus the solubility product is exceeded and the cations of group IV are precipitated as ZnS, NiS, MnS and CoS.