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  4. 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

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Q. 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

AIEEEAIEEE 2007Equilibrium

Solution:

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.