Question

The pH of $0.1M$ solution of a weak acid $\left(HA\right)$ is 4.50. It is neutralised with certain ammount of $NaOH$ solution to decrease the acid content to half of initial value. Calculate the pH of the resulting solution.

Answer 1

Answer: 8

$pH=pKa+log\frac{[Salt]}{[Acid]}$

$pH=pKa$

when weak acid is half neutralized by strong base.

For weak acid = $\text{pH}\text{=}\frac{\text{1}}{\text{2}}\left[\text{pKa}-\text{log}\text{C}\right]$

$\text{4}\text{.5}\text{=}\frac{\text{1}}{\text{2}}\text{[pKa}-{\text{log10}}^{-\text{1}}\text{]}$

$\text{9}-\text{1}\text{=}\text{pKa}\Rightarrow \text{pKa}\text{=}\text{8}$