Question

What happens to the pH of $500mL$ of a solution that is $0.1$ molar in sodium acetate and $0.1$ molar in acetic acid when $10mL$ of $0.1M$ sodium hydroxide is added?

• A. pH increases
• B. pH decreases
• C. pH doesn't get affected
• D. Resulting solution is not a buffer

Answer 1

The correct option is A pH increases

Initial $pH$ of solution will be:

$\Rightarrow p_1^H=p{K}_a+\log \frac{\left[sodiumacetate\right]}{\left[aceticacid\right]}=4.74+\log \frac{0.1}{0.1}=4.74$

Now, $10ml$ of $0.1M$ $NaOH$ neutralizes $0.1$ molar $C{H}_{3}COOH$ to form $C{H}_{3}COONa$ .

Final $\left[C{H}_{3}COONa\right]=0.2\times \frac{500}{510}M\approx 0.2M$

$pH$ will be driven by hydrolysis of $C{H}_{3}COONa$

$\Rightarrow p_2^H=7+1/2p{K}_a-1/2\log C$

$=7+\frac{4.74}{2}-\frac{1}{2}\log 0.2$

$=9.719$

$\therefore pH$ increases.