An aqueous blue-coloured solution of a transition metal sulphate reacts with `H_2S` I acidic medium to give a black precipitate A which is insoluble in warm auqous solution of KOH. The blue solution on treatment with KI in weakly acidic medium turns yellow and produces a white precipitate B. Identify the transition metal ion. write the chemical reactions involved in the formation of A and B.

Explanation

An aqueous blue-coloured solution of a transition metal sulphate reacts with `H_2S` in acidic medium to give a black precipitate A which is insoluble in warm aqueous solution of KOH. It must be the black precipitate of CuS because copper sulphate is also blue coloured. The blue solution on treatment with KI in a weak acidic medium turns yellow iodide is first formed which decomposes to give white cuprous iodide and iodine.
`CuSO_4+H_2Soverset("Acidic medium")toCuSdarr+H_2SO_4`
`CuSO_4+2KItoCuII_2+K_2SO_4` (Yellow soln.)
`2CuI_2tounderset((B) "white ppt. insoluble in H_2O")(Cu_2I_2darr+I_2)`