Question

$100$ mL of $0.02M$ benzoic acid $(p{K}_a=4.2)$ is titrated using $0.02MNaOH$. The pH after $50$ mL of $NaOH$ has been added is:

• A. 3.5
• B. 2.4
• C. 4.2
• D. 8.25

Answer 1

The correct option is C 4.2

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

Here, we add $50$ mL of $NaOH$.

$\left[salt\right]=\left[acid\right]=50\times 0.02$

So, $pH=p{K}_a=4.2$