Q3 / 120 · JEE Main 2007 · Chemistry
The $\mathrm{pKa}$ of a weak acid $(\mathrm{HA})$ is $4.5$. The $\mathrm{pOH}$ of an aqueous buffered solution of $\mathrm{HA}$ in which $50 \%$ of the acid is ionized is
- A$4.5$
- B$2.5$
- Correct answerC$9.5$
- D$7.0$
Solution
For buffer solution
$\begin{aligned}
& \mathrm{pH}=\mathrm{pK}_{\mathrm{a}}+\log \frac{[\text { Salt }]}{[\text { Acid }]} \\
& =4.5+\log \frac{[\text { Salt }]}{[\text { Acid }]}
\end{aligned}$
as $\mathrm{HA}$ is $50 \%$ ionized so $[\text{Salt}]$=$[\text{Acid}]$
$\begin{aligned}
& \mathrm{pH}=4.5 \\
& \mathrm{pH}+\mathrm{pOH}=14 \\
& \Rightarrow \mathrm{pOH}=14-4.5=9.5
\end{aligned}$
Hence (C) is correct.
Hence, the correct answer is option: 'C'.