Question

When aqueous $KCN$ is added to a solution of copper(II) sulphate, a white precipitate, soluble in an excess of $KCN$ is formed. No precipitate is formed when $H_{2}S$ is passed into it.

If the given statement is true enter 1 else enter 0.

Answer 1

When aqueous $KCN$ is added to a solution of copper(II) sulphate, a white precipitate, soluble in an excess of $KCN$ is formed.
It is due to formation of a stable complex which does not ionise to give free $C{u}^{2+}$ that can give precipitate with $H_{2}S$.
$\underset{free}{C{u}^{2+}}+S^{2-}⟶CuS\downarrow$
$CuS{O}_4+2KCN⟶\underset{whiteppt}{Cu(CN{)}_2}\downarrow +K_{2}S{O}_4$
$2Cu(CN{)}_2⟶2CuCN+\underset{cyanogen}{(CN{)}_2}$
$CuCN+3KCN⟶k_3\underset{stable}{\left[Cu\right(CN{)}_4]}$
$K_3\left[Cu\right(CN{)}_4]\rightleftharpoons 3K^{+}[Cu(CN{)}_4{]}^{3-}$
$\left[Cu\right(CN{)}_4{]}^{3-}⟶$ no ionisation
Hence, no precipitate is formed when $H_{2}S$ is passed into it.