Question

When ammonium chloride and ammonium hydroxide are added to a solution containing both ${Al}^{3+}$ and ${Ca}^{2+}$ ions, which ion is precipitated first and why?

Answer 1

When ammonium chloride and ammonium hydroxide are added to a solution containing both $A{l}^{3+}$ and $C{a}^{2+}$ ions, $A{l}^{3+}$ gets precipitated first as $Al(OH{)}_3$. The solubility product of $Al(OH{)}_3$ is much lower than the solubiltiy product of $Ca(OH{)}_2$.

$N{H}_{4}Cl$ suppreses the dissociation of $N{H}_{4}OH$ which results in very low hydroxide ion concentration. At such a low hydroxide concentration, the ionic product of $Al(OH{)}_3$ exceeds its solubility product which results in precipitation.

Note: $C{a}^{2+}$ ions are precipitated as $CaC{O}_3$ with the reagent ammonium carbonate $+$ ammonium hydroxide $+$ ammonium chloride.