Question

The $p{K}_a$ of a weak acid $\left(HA\right)$ is 4.5. The $pOH$ of an aqueous buffered solution of $HA$ in which 50% of the acid is ionized is:

• A. 4.5
• B. 2.5
• C. 9.5
• D. 7

Answer 1

The correct option is C 9.5

For buffer solution
$pH=p{K}_a+log\frac{\left[Salt\right]}{\left[Acid\right]}$
$pH=4.5+log\frac{\left[Salt\right]}{\left[Acid\right]}$
The reaction is $HA\left(acid\right)\rightleftharpoons A^{-}\left(salt\right)+H^{+}$ .
As the acid is 50% ionized, then same amount of salt is formed.
Therefore, [Salt] = [Acid]
$pH=4.5pH+pOH=14pOH=14-4.5=9.5$