Question

The $pK_a$ of a weak acid (HA) is 4.5. The pOH of an aqueous buffered solution of HA in which $50\%$ of the acid is ionized is

• A. 4.5
• B. 2.5
• C. 9.5
• D. 7

Answer 1

Answer: (C)

9.5

Answer 2

For buffer solution $pH=pK_{a}+log \frac{\left[Salt\right]}{\left[Acid\right]}$ $=4.5+log \frac{\left[Salt\right]}{\left[Acid\right]}$ as $HA$ is $50\%$ ionized so $\left[Salt\right] = \left[Acid\right]$ $pH = 4.5$ $pH + pOH = 14$ $\Rightarrow pOH = 14 - 4.5 = 9.5$ Hence (3) is correct.