Question

A solution consists of $1$ mole each of $C{H}_{3}COOH$ and $C{H}_{3}COONa$. The $pKa$ of acetic acid is $4.8$. What is the $pH$ of the solution, when $0.1$ mole of $NaOH$ is added?

• A. $4.8$
• B. Slightly greater than $4.8$
• C. Slightly less than $4.8$
• D. $7$

Answer 1

Answer: (B)

Slightly greater than $4.8$

When equimolar amounts of acetic acid and sodium acetate are present, $pH=pKa=4.8$.

When $0.1$ mole of $NaOH$ is added, some sodium acetate will be formed and some acetic acid will be neutralized.
$pH=p{K}_a+log\frac{C{H}_{3}COONa}{C{H}_{3}COOH}$
Thus, the term $\frac{C{H}_{3}COONa}{C{H}_{3}COOH}$ will be slightly greater than $1$ and $pH$ will be slightly greater than $4.8$.