Question

A certain buffer solution contains $X^{-}$ is twice the concentration of HX. If $K_a$ for HX is ${10}^{-6},$ then pH of buffer is

• A. 7
• B. 4.2
• C. 6.3
• D. 5.8

Answer 1

The correct option is C 6.3

Given, $K_a$ for $HX$ is ${10}^{-6}$
Let the concentration of $HX$ is $xM$
Then according to question,
concentration of its conjugate base $\left[X^{-}\right]=2xM$
Using the Henderson-Hasselbalch equation for weak acid:
$pH=p{K}_a+log\left[\frac{\left[\text{conjugate base}\right]}{\left[\text{acid}\right]}\right]$
$pH=-log{K}_a+log\left[\frac{\left[X^{-}\right]}{\left[HX\right]}\right]$
$pH=-log\left({10}^{-6}\right)+log\left[\frac{2x}{x}\right]$
$pH=6+log2$
$pH=6+0.301$
$pH=6.301$