Question

When ammonium chloride is added to ammonia solution, the pH of the resulting solution will be:
(A) Increased
(B) Seven
(C) Decreased
(D) Not changed

Answer 1

Hint: Ammonium chloride is a salt formed after the neutralization reaction of a weak base and a strong acid that at $N{H_4}OH$ (ammonium hydroxide) and $HCl$ (hydrochloric acid)
Whereas, ammonium solution is a weak basic solution.

Complete step by step answer:
As we know, $N{H_4}Cl$ is a salt, upon dissociation it will from two ions, that are $N{H_4}^ + $ and $C{l^ - }$
$N{H_4}Cl \rightleftharpoons N{H_4}^ + + C{l^ - }$
Similarly, upon dissociation ammonia solution will form two ions $N{H_4}^ + $and $C{l^ - }$
$N{H_3}\left( {aq} \right) \rightleftharpoons N{H_4}^ + + O{H^ - }$
As we can see in both the reactions, $N{H_4}^ + $ is a common ion. Hence, a common ion effect will come in charge here.
According to the common ion effect, the ionization of one electrolyte gets suppressed. When another electrolyte has the same ion is present in the solution. The same ions are known as common ions.
Hence, in the above reaction the ions put forward by ammonium chloride will suppress the ionization of ammonia solution. Due to suppression of ionization. The reaction will proceed backyards and thus the production of $O{H^ - }$ ions will be less. As production of $O{H^ - }$ ions will be less, the concentration of ${H^ + }$ ions in the solution will increase. Therefore, the $pH$ of the solution will decrease due to high concentration of ${H^ + }$ ions.
The common ion effect is considered as a consequence of equilibrium law or Le Chatlier’s principle.
This effect is used for obtaining potable water from underground water. It is also used in toothpaste manufacturing.

So, the correct answer is Option C .

Note:
The common ion effect is commonly shown by the elements of d-block. The d-block elements are capable of forming complex ions. For example, $CuC{l_2}$ compound is converted into $CuC{l_2}$ so that it can form a complex ion by common ion effect and become soluble in water.