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In the separation of Cu2+ and Cd2+ in II group qualitative analysis of cations tetrammine copper (II — Qualitative and Quantitative Analysis Chemistry Question

Question

In the separation of Cu2+ and Cd2+ in II group qualitative analysis of cations tetrammine copper (II) sulphate and tetrammine cadmium (II) sulphate react with KCN to form the corresponding cyano complexes. Which one of the following pairs of the complexes and their relative stability enables the separation of Cu2+ and Cd2+ ?

Answer: A

💡 Solution & Explanation

K3[Cu(CN)4] is a highly stable (perfect) complex, so it doesn't provide enough Cu+ to precipitate with H2S. K2[Cd(CN)4] is less stable (imperfect) and dissociates enough Cd2+ to form yellow CdS precipitate with H2S.

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