Question

The pKa of a weak acid (HA) 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

Given, pKa(HA) = 4.5
For buffer solution, Henderson-Hasselbalch equation is given by
$pH=p{K}_a+log\frac{\left[Salt\right]}{\left[Acid\right]}$
$pH=4.5+log\frac{\left[A^{-}\right]}{\left[HA\right]}$
As HA is 50% ionized,
Therefore, [Salt] = [Acid]
$pH=p{K}_a+log1$
pH = 4.5
Also, $pH+pOH=14$
$pOH=14-pH$
$pOH=14-4.5=9.5$