Question

What is the net effect of the common ion?
(A) It reduces the solubility of the solute in the solution.
(B) It increases the solubility of the solute in the solution.
(C) It causes more products to be made.
(D) It causes more ion to form.
(E) There is no net effect overall.

Answer 1

In the above question, it is asked about the net effect of common ion. Addition of common ion indicates addition of ion which is present on either reactant or product side. This prevents the other ions from dissolving.

Complete step by step solution:
We know that if we have a solution and solute in equilibrium then adding a common ion (an ion that is common with the dissolving solid) dissolves itself and hence, hinders the solubility of the solute present. This is according to Le Chatelier’s principle which states that if a dynamic equilibrium is disturbed by changing the condition, it tries to attain its equilibrium by counteracting the change.
As solute and the compound added both have the same ion, the compound and solute both try to dissolve in the solvent. Hence, the solubility of the solute in the solution decreases.
For example, ammonium hydroxide which is a weak electrolyte ionizes to a small extent to give ammonium ions and hydroxide ions.
${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}} \rightleftharpoons {\text{NH}}_{\text{4}}^{\text{ + }}{\text{ + O}}{{\text{H}}^{\text{ - }}}$
So, if a strong electrolyte like ${\text{NaOH}}$ is added which furnishes hydroxide ions then it suppresses the equilibrium for the dissociation of ammonium hydroxide.
${\text{NaOH}} \rightleftharpoons {\text{N}}{{\text{a}}^{\text{ + }}}{\text{ + O}}{{\text{H}}^{\text{ - }}}$ .
So, the correct option is option A.

Note:
Scientists have taken advantage of common ion property while purifying water. In areas where water sources are high in chalk or limestone, drinking water contains excess calcium carbonate. In the water treatment process, sodium carbonate salt is added to precipitate the calcium carbonate. A very pure and finely divided precipitate of calcium carbonate is generated which is used in the manufacture of toothpaste.