If H_2S gas is passed into a solution of Cu^ 2+ , Cd^ 2+ having excess of KCN

JEE Advanced Chemistry question

If gas is passed into a solution of , having excess of KCN

(A)CuS and CdS both are precipitated
(B)Soluble complex and are formed and no effect of passing gas
(C)Soluble complex and are formed, of which CdS is precipitated as yellow pptCorrect
(D)Soluble complex and are formed of which CuS is precipitated as black ppt

Answer: (C)

Solution

Excess reduces to and complexes both metals as and . The cyanocuprate(I) complex is extremely stable (very low free concentration), so passing cannot exceed the solubility product of CuS and no precipitate of copper sulphide forms. The cadmium

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Question ID VK-159624