Q10 / 160 · AP EAMCET 2019 (23 Apr Shift 1) · Chemistry
If the \(\mathrm{pH}\) of a buffer solution containing \(0.1 \mathrm{M}\) of monoacidic base and \(0.01 \mathrm{M}\) of its salt is 10.5 , the \(\mathrm{p} K_a\) of conjugate acid is
- A9.5
- B4.5
- C3.5
- Correct answerD11.5
Solution
Key Idea From Henderson-Hesselbalch equation
\(p H=\mathrm{p} K_a+\log \frac{[\text { salt] }}{\text { [acid] }}\)
\(\mathrm{pH}\) of a buffer solution \(=10.5\)
\(\mathrm{p} K_a\) of this buffer solution can be calculated as:
\(\begin{aligned}
\Rightarrow 10.5 & =\mathrm{p} K_a+\log \frac{[0.01]}{[0.1]} \\
10.5 & =\mathrm{p} K_a+\log 10^{-1} \\
10.5 & =\mathrm{p} K_a-1 \\
\mathrm{p} K_a & =11.5
\end{aligned}\)
Hence, the correct answer is option: 'D'.