Question

What is the pH of the solution when $0.2mole$ of $HCl$ is added to one litre of a solution containing $0.1M$ each of $C{H}_{3}COOH$ and acetate ion?
[Assume that the volume is 1 litre, $K_a=1.8\times {10}^{-5}$ ]

• A. $pH=1$
• B. $pH=2$
• C. $pH=4.57$
• D. None of the above

Answer 1

The correct option is A $pH=1$

$\begin{array}{cccccccc}& C{H}_{3}CO{O}^{-}& +& HCl& \to & C{H}_{3}COOH& +& C{l}^{-}\\ \text{Initial moles}& 0.1& & 0.2& & 0& & 0\\ \text{Moles after reaction}& 0& & 0.1& & 0.1& & 0.1\end{array}$
$\text{Total moles of}C{H}_{3}COOH=0.1+0.1=0.2$
Since $C{H}_{3}COOH$ is weak acid and $HCl$ is stong acid. In solution, both strong acid $\left(HCl\right)$ and weak acid $\left(C{H}_{3}COOH\right)$ are present, therefore the dissociation of $C{H}_{3}COOH$ is suppressed by the strong acid $\left(HCl\right)$, due to common ion effect.

So, here we can neglect the $H^{+}$ ion concentration from the $C{H}_{3}COOH.$ We can consider the conc. of $H^{+}$ from the strong acid $HCl$ only.

$\left[HCl\right]=\left[H^{+}\right]=0.1$
$pH=-log\left[H^{+}\right]=-log\left(0.1\right)=1$