Question

When sodium acetate, ${{C}}{{{H}}_3}{{COONa}}$ is added to aqueous solution of acetic acid, ${{C}}{{{H}}_3}{{COOH}}$,
A. the ${{pH}}$ value becomes zero.
B. the ${{pH}}$ value remains unchanged.
C. ${{pH}}$ value decreases.
D. ${{pH}}$ value increases.

Answer 1

We know that ${{pH}}$ is calculated by taking the logarithm of the hydrogen ion concentration. Sodium acetate is a salt obtained when acetic acid is reacted with any hydroxides like sodium hydroxide or potassium hydroxide. Acetic acid is a very weak acid and sodium acetate is a very strong base.

Complete answer:
The mixture of both solutions makes up an acidic buffer. Acidic buffer is a solution which has a ${{pH}}$ less than $7$ and it is a mixture of a weak acid and its conjugate base.
Another name for sodium acetate is sodium ethanoate and that for acetic acid is ethanoic acid. The common ion in both of these compounds is ethanoate ion.
Generally acetic acid dissociates into hydrogen ion and acetate ion in aqueous solution. Since sodium acetate is very strong, it dissociates into ions completely in solution. Also, we know that acetic acid does not dissociate completely in solution. Thus ionization of acetic acid is suppressed by the sodium acetate because both of them have a common ion, i.e. ${{C}}{{{H}}_3}{{CO}}{{{O}}^ - }$ ion. Due to this, the concentration of sodium increases. Thus the solution becomes a basic solution. And we know that a basic solution has increased ${{pH}}$ value.
Thus we can say that when sodium acetate is added to aqueous solution of acetic acid, ${{pH}}$ value increases.

Hence, the correct option is D.

Note:
At first sight, we may get confused that the ${{pH}}$ value remains unchanged. But this is not correct because if we are adding a strong acid with a weak acid, the ${{pH}}$ value may remain unchanged. When the weak acid is reacted with another acid, there occurs incomplete dissociation. In this mixture, the ratio of sodium acetate to acetic acid is $10$.