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Q.
Copper sulphate dissolves in excess of $KCN$ to give
AIPMTAIPMT 2006The d-and f-Block Elements
Solution:
When copper sulphate dissolves in potassium cyanide, then following reaction takes place:-
$CuSO _{4}+2 KCN \rightarrow K _{2} SO _{4}+ Cu ( CN )_{2}$
Now, since $Cu ( CN )_{2}$ is unstable, it undergoes decomposition as:-
$Cu ( CN )_{2} \rightarrow CuCN +( CN )_{2}$
$Now , CuCN +3 KCN \rightarrow K _{3}\left[ Cu ( CN )_{4}\right]$
that is $\left[ Cu ( CN )_{4}\right]^{3-}$