Question

What is the $pH$ of a solution when $0.2$ moles of $HCl$ acid is added to one litre solution containing $1M$ each of acetic acid and acetate ion?

• A. $pH=4.57$
• B. $pH=5.57$
• C. $pH=4.77$
• D. $pH=4.67$

Answer 1

The correct option is A $pH=4.57$

Given that

[Acetic acid]$=1M$, $\left[{CH}_{3}CO{O}^{-}\right]=1M$

Addition of $0.2$ mole of $HCl$ to this buffer leads to the following changes:

$\underset{0.20.0}{HCl}+\underset{10.8}{{CH}_{3}CO{O}^{-}}⟶\underset{11.2}{{CH}_{3}COOH}+\underset{00.2}{{Cl}^{-}}$

$\therefore$ New [Acetic Acid]$=1.2$; [Acetate]$=0.8$

$pH=-log{K}_a+log\frac{\left[C{H}_{3}CO{O}^{-}\right]}{\left[C{H}_{3}COOH\right]}$

$pH=-\log \left(1.8\times {10}^{-5}\right)+\log \frac{0.8}{1.2}=4.57$