Question

$500mL$ of $0.2M$ aqueous solution of acetic acid is mixed with $500mL$ of $0.2MHCI$ at ${25}^{o}C$. If $6g$ of $NaOH$ is added to the above solution, determine the final pH (assuming no change in volume on mixing. $K_a$ of acetic acid is $1.75\times {10}^{-5}mol{L}^{-1}$. (write the value to the nearest integer)

Answer 1

The number of moles of NaOH present $=\frac{wt.}{mol.wt.}=\frac{6}{40}=0.15$ moles.
Out of this, 0.1 moles will be neutralized with HCl and 0.05 moles will be neutralized with acetic acid.

Hence, $\left[C{H}_{3}COOH\right]=0.1-0.05=0.05M$ and $\left[C{H}_{3}COONa\right]=0.05M$

$p{K}_a=-lo{g}_{10}{K}_a=-lo{g}_{10}(1.75\times {10}^{-5})=4.75$ [Given: $K_a=1.75\times {10}^{-5}mol{L}^{-1}$]

$pH=pKa+log\frac{C{H}_{3}COONa}{C{H}_{3}COOH}=4.75+log\frac{0.05}{0.05}=4.75+log1=4.75+0=4.75$

The nearest integer value of $pH=5$
The correct answer is $5$.