Question

A solution contains $0.4M$ ${CH}_{3}COOH$ and $0.2M$ ${CH}_{3}COONa$. Calculate the concentration of $H^{+}$ ions. The ionisation constant of acetic acid is $1.8\times {10}^{-5}$.

Answer 1

Using Henderson's equation,

$pH=p{K}_a+\log \frac{\left[Salt\right]}{\left[Acid\right]}$

$pH=5-log\left(8\right)+\log \frac{\left[0.2\right]}{\left[0.4\right]}=4.44$

So $\left[H^{+}\right]=3.6\times {10}^{-5}M$