Question

$500\, mL$ of $0.2\, M$ aqueous solution of acetic acid is mixed with $500 \,mL$ of $0.2\, M\,HCl$ at $250^{\circ} C$.
(i) Calculate the degree of dissociation of acetic acid in the resulting solution and $pH$ of the solution.
(ii) If $6\, g$ of $NaOH$ is added to the above solution, determine the final $pH $ (assuming there is no change in volume on mixing, $K_{a}$ of acetic acid is $1.75 \times 10^{-1} \,mol / L$).

Answer 1

If no $HCl$ is present,
${[ HCl ]=\frac{0.2}{2}=0.10 M }$
${\left[ CH _{3} COOH \right]=0.10 M }$
The major contributor of $H ^{+}$in solution is $HI$
$K_{a}=\frac{C \alpha(0.1)}{C(1-\alpha)}=1.75 \times 10^{-5} $
$\alpha=1.75 \times 10^{-4}$
(ii) $m\,mol$ of $NaOH$ added $=\frac{6}{40} \times 1000=150$
$m\,mol$ of $HCl =500 \times 0.2=100$
$m\,mol$ of $CH _{3} COOH =500 \times 0.2=100$
After neutralisation, mmol of $CH _{3} COOH =50$
mmol of $CH _{3} COONa =50$
$pH = p K_{a}=4.75$