Question

An aqueous blue coloured solution of a transition metal sulphate reacts with H₂S in acidic medium to give a black precipitate A, which is insoluble in warm aqueous solution of KOH. The blue solution an treatment with KI is 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.

Answer 1

The transitional metal is Cu²⁺. The compound is CuSO₄.5H₂O. It dissolves in water to give blue coloured solution due to presence of Cu²⁺ (d⁹-configuration). On passing H₂S in acidic medium in such solution, the black precipitate of CuS is obtained which is not soluble in aqueous KOH (warm) solution.

On addition of KI solution in aqueous solution of CuSO₄, it produces yellow coloured solution of CuI₂, which is decomposed in white ppt. of Cu₂I₂ and liberate I₂.