Question

On adding excess of $N{H}_{4}OH$ to copper sulphate solution

• A. A deep blue solution is obtained
• B. A blue precipitate of $Cu(OH{)}_2$ is obtained
• C. A black precipitate of CuO is obtained
• D. No change takes place

Answer 1

Answer: (A)

With ammonium solution, $CuS{O}_4$ forms a stable blue complex. First it forms a precipitate of $Cu(OH{)}_2$ which dissolves in excess of ammonia solution forming a deep blue coloured complex.
${\mathrm{CuSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Cu}(\mathrm{OH}{)}_2+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$
$\mathrm{Cu}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_4\mathrm{OH}+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\to \mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{SO}}_4+4{\mathrm{H}}_2\mathrm{O}$ Tetraamminecupricsulphate