Question
Which compound is formed when excess of $$KCN$$ is added to aqueous solution of copper sulphate?
A.
$$Cu{\left( {CN} \right)_2}$$
B.
$${K_2}\left[ {Cu{{\left( {CN} \right)}_4}} \right]$$
C.
$$K\left[ {Cu{{\left( {CN} \right)}_2}} \right]$$
D.
$${K_3}\left[ {Cu{{\left( {CN} \right)}_4}} \right]$$
Answer :
$${K_3}\left[ {Cu{{\left( {CN} \right)}_4}} \right]$$
Solution :
$$\eqalign{
& CuS{O_4} + 2KCN \to Cu{\left( {CN} \right)_2} + {K_2}S{O_4} \cr
& 2Cu{\left( {CN} \right)_2} \to C{u_2}{\left( {CN} \right)_2} + {\left( {CN} \right)_2}\left( {{\text{Cyanogen}}} \right) \cr
& C{u_2}{\left( {CN} \right)_2} + 6KCN \to 2{K_3}\left[ {Cu{{\left( {CN} \right)}_4}} \right] \cr} $$