Question

If $N{H_4}OH$ in presence of $N{H_4}Cl$ is added to a solution containing $A{l_2}{(S{O_4})_3}$and $Mg S{O_4}$ , which of the following will precipitate
(A) $Al{(OH)_3}$ only
(B) $Mg{(OH)_2}$ only
(C) $Al{(OH)_3}$ and $Mg{(OH)_2}$
(D) None of these

Answer 1

Hint: Ammonium hydroxide and ammonium chloride are buffer solutions. Ammonium hydroxide provides ion but since it is a weak base it does not ionise completely. It decreases the concentration of hydroxide ions and does not let higher group cations to precipitate.

Complete step by step solution:
Ammonium hydroxide and ammonium are buffer pairs. Ammonium chloride being a strong electrolyte decomposes completely. The ammonium ions present in ammonium chloride suppress the ionisation of ammonium hydroxide due to a phenomenon called the common ion effect. It decreases the hydroxide ion concentration and higher group cations don’t get precipitated.
Here the sparingly soluble hydroxides are precipitated by the addition of ammonium hydroxide containing $A{l^{3 + }}$ions. So precipitate of aluminium hydroxide is found.
Magnesium ions are readily soluble in ammonium salt solutions.No precipitate is formed in presence of ammonium salts since concentration is reduced due to the phenomenon of common ion effect.
Therefore only $Al{(OH)_3}$ is precipitated when ammonium hydroxide is added in presence of ammonium chloride.

Hence, the correct answer is option A.

Note: Ammonium salts dissolve the ammonium ions completely to prevent its precipitation. In the given reaction ammonium hydroxide and ammonium chloride helps in maintaining the pH. Here aluminium hydroxide is insoluble so it precipitates and the remaining are soluble so they don’t give a precipitate.