Copper sulphate dissolves in excess of KCN to give
(3) [Cu(CN)4]3-
CuSO4 + 2KCN Cu(CN)2 + K2SO4
Cu(CN)2 is unstable and decomposes:
2Cu(CN)2 2CuCN + (CN)2
With excess KCN, CuCN dissolves forming a soluble complex:
CuCN + 3KCN K3[Cu(CN)4]
∴ The final species is [Cu(CN)4]3- (copper in +1 state).
Answer: (3)
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