Question

The addition of sodium carbonate in pure water causes:
(A) An increase in the hydrogen ion concentration
(B) A decrease in hydroxyl ion concentration
(C) No change in $pH$
(D) An increase in $pH$

Answer 1

Hint: pure water has equal concentration of hydrogen ion and hydroxyl ion. Sodium carbonate on hydrolysis produces sodium hydroxide and carbonic acid. Sodium Hydroxide dissociates completely to produce hydroxyl ions and carbonic acid does not dissociate completely to produce hydrogen ions.

Complete solution step by step:
-Pure water has equal concentrations of hydroxyl ions and hydrogen ion. Concentration of hydroxyl ion and hydrogen ion is ${{10}^{-7}}$.Pure water has $pH$ equal to 7.
-when sodium carbonate is added to pure water, sodium carbonate undergoes hydrolysis.
-Hydrolysis of salt is defined as when ions of salt react with ions of water to produce acidity or alkalinity or neutral solution.
-When sodium carbonate undergoes hydrolysis, Hydroxyl ion reacts with Sodium ion to produce sodium hydroxide. As sodium hydroxide is a strong base it dissociates completely to produce hydroxyl ions.
-Hydrogen ions react with carbonate ions to produce carbonic acid as carbonic acid is a weak acid that does not dissociate completely to produce hydrogen ions.
-As hydroxyl ions are more than hydrogen ions, there is an increase in $pH$from 7.

The addition of sodium carbonate in pure water causes (C) increase in $pH$.

Note: Pure water has equal concentration of hydroxyl ions and hydrogen ions. When salt is a combination of strong acid and weak base, addition of salt in pure water will produce acidic solution. when salt is a combination of strong base and weak acid, addition of salt in pure water will produce a basic solution.