. During the qualitative analysis of a mixture containing Cu^2+ and Zn^2+ ions, H base 2S gas is passed through an acidified solution containing these ions in order to test Cu^2+ alone.Explain briefly.

The solubility products of CuS and ZnS are

K base sp(CuS) = 10^-38 and K base sp (ZnS) = 10^-22

Since K base sp (CuS) << K base sp (ZnS), very small concentration of S^2 is sufficient to cause the precipitation of Cu^2+ ions. In order to have very small concentration of S^2- ions, acidic medium is used. Due to the common ion H^+, the ionization of H base 2S is suppressed:

H base 2S ? 2H^+ +S^2-

The available concentration of S^2- ions in acidic medium causes only the precipitation of CuS and not that of ZnS.