Passing `H_(2)S` gas into a mixture of `Mn^(2+), Ni^(2+), Cu^(2+)` and `Hg^(2+)` ions in an acidified aqueous solution precipitates

By passing H_(2)S gas in acidified KMnO_(4) , we get

By passing H_(2)S gas in acidified KMnO_(4) solution, we get

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