Question

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

• A. 4.5
• B. 2.5
• C. 9.5
• D. 7

Answer 1

The correct option is C 9.5

For buffer solution, Henderson equation is
$pH=p{K}_a+\log \frac{salt}{acid}pH=4.5+\log \frac{salt}{acid}$
As $HA$ is 50% ionised,
[salt] = [acid]
$\therefore$
$pH=4.5+\log 1$
$pH=4.5$
$pH+pOH=14pOH=14-4.5=9.5$