Which pair will show common ion effect?

The correct option is C

${\mathrm{NH}}_4\mathrm{OH}+{\mathrm{NH}}_4\mathrm{Cl}$

Explanation for correct option

(C) ${\mathbf{NH}}_{\mathbf{4}}\mathbf{OH}\mathbf{+}{\mathbf{NH}}_{\mathbf{4}}\mathbf{Cl}$

Common ion effect

• The term "common-ion effect" describes the decrease in solubility of an ionic precipitate caused by the addition of a soluble compound that has an ion with the precipitate to the solution.
• The common ion effect is seen when weak and strong electrolytes are mixed.
• Solutions containing both Sodium chloride $\left(\mathrm{NaCl}\right)$ and Silver chloride $\left(\mathrm{AgCl}\right)$ also contain a common ion, ${\mathrm{Cl}}^{-}$ ion.
• The equilibrium is as follows:

$\mathrm{AgCl}\rightleftharpoons {\mathrm{Ag}}^{+}+{\mathrm{Cl}}^{-}$

• The equilibrium will be moved to the left to generate more solid $\mathrm{AgCl}$.
• As a result, the solubility of $\mathrm{AgCl}$ will decrease.

Common ion effect in ${\mathbf{NH}}_{\mathbf{4}}\mathbf{OH}\mathbf{+}{\mathbf{NH}}_{\mathbf{4}}\mathbf{Cl}$

• Since ${\mathrm{NH}}_4\mathrm{OH}$ is a weak electrolyte and ${\mathrm{NH}}_4\mathrm{Cl}$ is a strong electrolyte, the common ion effect is seen.
• When ${\mathrm{NH}}_4\mathrm{OH}$ is combined with ${\mathrm{NH}}_4\mathrm{Cl}$, due to the common ${{\mathrm{NH}}_4}^{+}$ ion, dissociation of ${\mathrm{NH}}_4\mathrm{OH}$ is reduced.

Explanation for incorrect options

(A) ${\mathbf{BaCl}}_{\mathbf{2}}\mathbf{+}\mathbf{Ba}{\left({\mathrm{NO}}_3\right)}_{\mathbf{2}}$

• ${\mathrm{BaCl}}_2$ and $\mathrm{Ba}{\left({\mathrm{NO}}_3\right)}_2$ are strong electrolytes.
• Hence ${\mathrm{BaCl}}_2+\mathrm{Ba}{\left({\mathrm{NO}}_3\right)}_2$ does not show the common ion effect.

(B) $\mathbf{NaCl}\mathbf{+}\mathbf{HCl}$

• $\mathrm{NaCl}$ and $\mathrm{HCl}$ are strong electrolytes.
• Hence $\mathrm{NaCl}+\mathrm{HCl}$ does not show the common ion effect.

(D) $\mathbf{AgCN}\mathbf{+}\mathbf{KCN}$

• $\mathrm{AgCN}$ and $\mathrm{KCN}$ are strong electrolytes.
• Hence $\mathrm{AgCN}+\mathrm{KCN}$ does not show the common ion effect.

Therefore, the correct option is (C) ${\mathbf{NH}}_{\mathbf{4}}\mathbf{OH}\mathbf{+}{\mathbf{NH}}_{\mathbf{4}}\mathbf{Cl}$.