Question

A certain buffer solution contains equal concentration of $X^{-}$ and $\text{HX}$ . The $K_{a}$ for $\text{HX}$ is $10^{- 8}$ . The $\text{pH}$ of the buffer is

Answer 1

The Henderson equation is

$pH=pK_{a}+log \frac{\left[C o n j u g a t e \text{base}\right]}{\left[A c i d\right]}$

$=-log K_{a}+log ⁡ \frac{\left[X^{-}\right]}{\left[H X\right]}$

Hence, $\left[\right.HX\left]\right.=\left[X^{-}\right]$

$=-log \left(\right.10^{- 8}\left.\right)+log ⁡ 1$

or pH = 8.0.