Question

A solution when diluted with $H_{2}O$ and boiled, gives a white precipitate. On addition of excess ${NH}_{4}Cl/{NH}_{4}OH$, the volume of precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which dissolves in ${NH}_{4}OH/{NH}_{4}Cl$ .

• A. ${Zn\left(OH\right)}_2$
• B. ${Al\left(OH\right)}_3$
• C. ${Mg\left(OH\right)}_2$
• D. ${Ca\left(OH\right)}_2$

Answer 1

The correct option is A ${Zn\left(OH\right)}_2$

Zinc(II) ions on reaction with ammonia solution, forms white gelatinous precipitate of zinc hydroxide which is readily soluble in excess of reagent.

$Z{n}^{2+}+2N{H}_3+2H_{2}O\to Zn(OH{)}_2\downarrow (whitegelatinousppt)+2N{H}_4^{+}$

$Zn(OH{)}_2\downarrow +4N{H}_3\to [Zn(N{H}_3{)}_2{]}^{2+}+2O{H}^{-}$

Option A is correct.