Question

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

• A. $Zn(OH{)}_2$
• B. $ZAl(OH{)}_3$
• C. $Mg(OH{)}_2$
• D. $ZCa(OH{)}_2$

Answer 1

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

Option (A) is correct.

$\begin{array}{ccccccc}Z{n}^{2+}& +& 2H_{2}O& ⟶& Zn(OH{)}_2\downarrow & +& 2H^{+}\\ & & & & White& & \end{array}$
$\begin{array}{ccccccccc}Zn(OH{)}_2& +& 2N{H}_{4}Cl& +& 2N{H}_{4}OH& ⟶& \left[Zn\right(N{H}_3{)}_4]C{l}_2& +& 4H_{2}O\\ & & & & & & \left(Soluble\right)& & \end{array}$
$Zn(OH{)}_2$ precipitate dissolves in excess of $N{H}_{4}OH$ in the presence of $N{H}_{4}Cl$ to form tetrammine soluble complex.