Question

Which of the following pairs will show a common ion effect?
A) $NaCN + KCN$.
B) $NaCl + HCl$
C) $N{H_4}OH + N{H_4}Cl$
D) $BaC{l_2} + Ba{\left( {N{O_3}} \right)_2}$

Answer 1

The common ion effect is usually used to describe an effect on an equilibrium that involves a substance which adds an ion that is a part of the equilibrium. The common ion effect is considered to be a consequence of Le Chatlier’s principle (the equilibrium law).

Complete answer:
The effect that suppresses an ionization of the electrolyte when the other electrolyte (containing an ion which is present in the first electrolyte also) is added. When the solution of a weak electrolyte is added or mixed with the solution of strong electrolyte and that provides an ion which is common to that provided by a weak electrolyte and it is called a common ion effect.
For the better explanation, let’s take an example of silver chloride solution. When sodium chloride is added to this solution, then the chlorine concentration increases and the equilibrium will shift to the left side and AgCl will form. The solubility of the silver chloride also decreases.
$AgCl\left( {aq} \right) \rightleftharpoons A{g^ + } + C{l^ - }$
$NaCl\to Na^+ + Cl^-$
In the given options, ammonium chloride is a strong electrolyte and ammonium hydroxide is a weak electrolyte. Ammonium chloride produces ions that are common to that provided by the ammonium hydroxide.
$N{H_4}OH \rightleftharpoons NH_4^ + + O{H^ - }$
$N{H_4}Cl \to NH_4^ + + C{l^ - }$
All other remaining pairs have a mixture of strong electrolyte that is added or mixed with the other strong electrolyte. So, that is why they do not show a common ion effect.

Thus, the correct answer is option C. i.e., $N{H_4}OH + N{H_4}Cl$ shows the common ion effect.

Note: The common ion effect is used to produce drinking water from the aquifers that contain chalks or limestone. In order to decrease the hardness of the water, sodium carbonate is added to the water. A common ion effect can also be used in the precipitation of calcium carbonate from the water with the help of sodium carbonate.