Question

For an aqueous solution of a weak electrolyte $C{H}_{3}COOH$, how does the presence of $C{H}_{3}COONa$ affect its degree of ionisation?

• A. The degree of ionisation remains same.
• B. The degree of ionisation decreases.
• C. The degree of ionisation increases.
• D. $C{H}_{3}COOH$ acts as a strong acid.

Answer 1

The correct option is B The degree of ionisation decreases.

Presence of other solutes:
When a substance is present in a solution, it may affect the dissociation of another substance.
This is due to common ion effect.
It is the decrease in the degree of ionization of the weak electrolyte, when a strong electrolyte with a common ion is added to the aqueous solution of a weak electrolyte.
For example - For a solution of weak electrolyte $C{H}_{3}COOH$ :
$C{H}_{3}COOH\left(aq\right)+H_{2}O\left(l\right)\rightleftharpoons C{H}_{3}CO{O}^{-}\left(aq\right)+H_3{O}^{+}\left(aq\right)$
when strong electrolyte with common ion $C{H}_{3}COONa$ is added,
$C{H}_{3}COONa\left(aq\right)\rightleftharpoons C{H}_{3}CO{O}^{-}\left(aq\right)+N{a}^{+}\left(aq\right)$
According to Le - Chatelier’s Principle,
Reaction goes backward when the concentration of products, (here $C{H}_{3}CO{O}^{-}$) increases.
Hence degree of ionization decreases.