Question

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

• A. $3.50,7$
• B. $4.2,7$
• C. $4.2,4.05$
• D. $4.2,8.25$

Answer 1

The correct option is C $4.2,4.05$

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

$=4.2+log\left(\frac{50\times 0.02}{50\times 0.02}\right)$

$=4.2$

$pH=\frac{1}{2}[{pK}_w-{pK}_a-logC]=\frac{1}{2}[14-4.2-log(2\times {10}^{-2}\left)\right]=\frac{1}{2}[14-4.2-1.699]=\frac{8.1}{2}pH=4.05$