Question

What colour precipitates are formed when aqueous ammonia reacts with:

Zinc sulphate

State the solubility of each precipitate in excess of ammonium hydroxide.

Answer 1

Reaction with zinc sulphate$\left({\mathrm{ZnSO}}_4\right)$:

• When ammonium hydroxide $\left({\mathrm{NH}}_4\mathrm{OH}\right)$ is treated with zinc sulphate solution, it forms a white gelatinous precipitate of $\mathrm{Zn}(\mathrm{OH}{)}_2$, which is soluble in excess of ammonium hydroxide.
• One mole of zinc sulphate reacts with two moles of ammonium hydroxide to form one mole of $\mathrm{Zn}(\mathrm{OH}{)}_2$ and one mole of ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$.
• ${\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)⟶\underset{(\mathrm{white}\mathrm{gelatinous})}{\mathrm{Zn}(\mathrm{OH}{)}_2\left(\mathrm{s}\right)}+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)$.

Therefore, white gelatinous precipitate is formed when aqueous ammonia reacts with zinc sulphate which is soluble in excess of ammonium hydroxide.