The solubility products of $\mathrm{Al}(\mathrm{OH})_3$ and $\mathrm{Zn}(\mathrm{OH})_2$ are $8.5

Question

The solubility products of $\mathrm{Al}(\mathrm{OH})_3$ and $\mathrm{Zn}(\mathrm{OH})_2$ are $8.5 	imes 10^{-23}$ and $1.8 	imes 10^{-14}$ respectively. If $\mathrm{NH}_4 \mathrm{OH}$ is added to a solution containing $\mathrm{Al}^{3+}$ and $\mathrm{Zn}^{2+}$ ions, then substance precipitated first is

Professor PC Thomas Classes · Answer

Required $[OH^-]$ for precipitation is lower for $Al(OH)_3$ due to its extremely low $K_{sp}$ compared to $Zn(OH)_2$.