Question

Solubility product of $Al{{(OH)}_{3}}$ and $Zn{{(OH)}_{2}}$ ​ are $8.5\times {{10}^{-23}}$ and \[1.8\times {{10}^{-14}}\] respectively. If both aluminium and zinc ions are present in a solution, which one will precipitate first on addition of ammonium hydroxide?
A. $Al{{(OH)}_{3}}$
B. $Zn{{(OH)}_{2}}$
C. $Mg{{(OH)}_{2}}$
D. $Ca{{(OH)}_{2}}$

Answer 1

Hint: A salt forms precipitate only when the product of the concentration of its cation and anion is greater than its solubility product. Solubility product of a salt depends on the number of ions formed in a reaction or the concentration of the ions at equilibrium.

Complete Step by Step Answer:
Aluminium hydroxide dissociates into aluminium cation and hydroxide ion. The dissociation is given as follows:
\[Al{{\left( OH \right)}_{3}}\rightleftharpoons A{{l}^{+3}}+3O{{H}^{-}}\]
Zinc hydroxide dissociates to give zinc ion and hydroxyl ion. The dissociation is given as follows-
\[Zn{{\left( OH \right)}_{2}}\rightleftharpoons Z{{n}^{+2}}+2O{{H}^{-}}\]

Ammonium hydroxide is a weak base. It dissociates into ammonium ions and hydroxyl ions in solution. The dissociation is as follows:
\[N{{H}_{4}}OH\rightleftharpoons N{{H}_{4}}^{+}+O{{H}^{-}}\]
Thus due to the addition of ammonium hydroxide it will show a common ion effect due to the presence of hydroxyl ion. So, the solubility of the salts decreases. Again solubility product of $Al{{(OH)}_{3}}$ and $Zn{{(OH)}_{2}}$ are $8.5\times {{10}^{-23}}$ and \[1.8\times {{10}^{-14}}\] respectively so, the solubility product of aluminium hydroxide is much less than zinc hydroxide. Thus aluminium hydroxide will precipitate first.
Thus the correct option is A.

Note: Precipitate is an insoluble solid substance that is formed from a solution. The process of forming precipitate is called precipitation. It is only formed when the concentration of the species in solution is higher in amount or the solubility of the species decreases due to common ion effect.