Question

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

• A. $3$
• B. $8$
• C. $11$
• D. $14$

Answer 1

Answer: (B)

$8$

As the value of $K_a$ is given to us, it is an indication that the given buffer is an example of an acidic buffer.

Hence, we use the following formula for calculating its $pH$.

$pH=-\log K_w+\log \frac{\left[Conjugatebase\right]}{\left[Acid\right]}$

$pH$=$p{K}_a$ + $lo{g}_{10}\frac{\left[X^{-}\right]}{\left[HX\right]}$$=-lo{g}_{10}{10}^{-8}$ + $lo{g}_{10}1$$=8$
(As the concentrations of $X^{-}$ and $HX$ are equal)