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When excess KCN is added to an aqueous solution of copper sulphate, which compound is formed?
- Cu(CN)2
- K2[Cu(CN)4]
- K[Cu(CN)2]
- K3[Cu(CN)4]
Correct answer: K3[Cu(CN)4]
Solution
CN- reduces Cu²+ to Cu⁺, which then forms a stable tetracyanoargentate-type complex [Cu(CN)4]³- with excess CN-. The potassium salt formed is K3[Cu(CN)4].
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