Question

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

• A. $7.0$
• B. $4.5$
• C. $2.5$
• D. $9.5$

Answer 1

The correct option is D $9.5$

$pH=p{K}_a+\log \frac{\left[Conjugatebase\right]}{\left[acid\right]}$

$pKa=4.5$

$\left[Conjugatebase\right]=\left[acid\right]$, as $50$% ionification

$pH=4.5+\log \frac{0.5}{0.5}$

$pH=4.5$

Then, $pOH=14-4.5=9.5$

Note: $(pH+pOH=14)$