Question

Acetic acid dissociates as, ${\text{C}}{{\text{H}}_3}{\text{COOH }} \to {\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ - }{\text{ + }}{{\text{H}}^ + }$ . If a little amount of sodium acetate is added to its aqueous solution then:
A)the acid dissociates further
B)the ${{\text{H}}^ + }$ ion concentration increases
C)the acid dissociation is suppressed
D)the equilibrium is unaffected

Answer 1

To solve this question, it is required to have knowledge about the common ion effect and how pH of the solution is affected. The common-ion effect refers to the decrease in solubility of an ionic precipitate by the addition to the solution of a soluble compound with an ion in common with the precipitate. pH of a solution is dependent on the concentration of hydrogen ions in a solution.

Complete step by step answer:
As we know that, the common ion effect occurs when a compound containing an ion which is common to one of the ion present in the solution is added to the solution, it results in decrease in solubility of that ion. This occurs according to the Le-Chatelier principle. When the solution dissociates and will be present at equilibrium the rate of forward and backward reaction will be the same. If a compound is added with a common ion, the concentration of that ion in the solution increases and thus, the equilibrium will shift in the opposite direction of the dissociation. Thus, the extra ions will precipitate in the solution.
In the question given, acetate ion is present in equilibrium. When sodium acetate is added to the solution, it will dissociate as:
${\text{C}}{{\text{H}}_3}{\text{COONa}} \to {\text{CHCO}}{{\text{O}}^ - }{\text{ + }}{\text{ N}}{{\text{a}}^ + }$
Thus, the concentration of acetate ions will increase in the solution. According to Le-Chartelier’s principle, the equilibrium will shift backwards. As a result, the concentrate of the acetate ion as well as the hydrogen ion will decrease. This means that the dissociation of the acid will be suppressed.
$\therefore$ The correct option is option C, i.e. the acid dissociation is suppressed.

Note: In this question, as the concentration of hydrogen ions will decrease, the pH of the solution will also increase. This means the solution will become less acidic. The solution of a weak acid and its salt is also known as an acidic buffer solution. This means that the pH of the solution will remain unaffected by addition of base in the solution until a certain amount.