Question

pH of 0.1 M solution of a weak acid (HA) is 4.50. It is neutralised with NaOH solution to decrease the acid content to half of its initial value. Calculate the pH of the resulting solution.

• A. 8.00
• B. 10.00
• C. 4.50
• D. 7.00

Answer 1

The correct option is A 8.00

As, $\left[H^{+}\right]=\sqrt{K_{a}C}$
$K_a=\frac{[H^{+}{]}^2}{\left[C\right]}$
$K_a=\frac{({10}^{-4.5}{)}^2}{0.1}=\frac{{10}^{-9}}{0.1}={10}^{-8}\Rightarrow p{K}_a=8$
$pH=p{K}_a+log\frac{\left[Salt\right]}{\left[Acid\right]}$
At half equivalence point, $\left[Salt\right]=\left[Acid\right]$
So, $pH=p{K}_a=8$