Question

The pKa of a weak acid (HA) is 4.5. The pOH of an aqueous buffered solution of 50% ionized HA will be:

• A. 4.5
• B. 2.5
• C. 9.5
• D. 7

Answer 1

Answer: (C)

9.5

For buffer solution:

$pH=pKa+log\frac{\left[Salt\right]}{\left[Acid\right]}$

$pH=4.5+log\frac{\left(Salt\right]}{\left[Acid\right]}$

As $HA$ is $50\%$ ionized so $\left[Salt\right]=\left[Acid\right]$

$pH=4.5+log\left(1\right)=4.5$

Now,

$pH+pOH=14$

$\therefore pOH=14-4.5=9.5$