Question

$20$ ml of $0.1$ M acetic acid is mixed with $10$ ml of sodium acetate. $p{K}_a$ of $C{H}_{3}COOH$ is $4.8$, then the concentration of sodium acetate, if $pH$ of the mixture is $4.8$ is:

• A. $0.1M$
• B. $0.4M$
• C. $0.2M$
• D. $1M$

Answer 1

Answer: (C)

$0.2M$

The expression for the pH of the acidic buffer solution is as given below.

$pH=p{K}_a+log\frac{\left[Salt\right]}{\left[Acid\right]}$

When $pH=p{K}_a$, $\left[Salt\right]=\left[Acid\right]$

$\left[Acid\right]=\frac{20\times 0.1}{30}=0.067$ M

Hence, in the mixture of acetic acid and sodium acetate $\left[Salt\right]=0.067$ M.

The total volume of the mixture is 30 ml. The volume of the original solution of sodium acetate is 10 ml.

Hence, in the original solution of sodium acetate $\left[Salt\right]=0.067\times \frac{30}{10}=0.2$ M.

Option C is correct.