Chemical and Ionic Equilibrium 2 Question 11
11. $\mathrm{p} K_{a}$ of a weak acid $(\mathrm{H} A)$ and $\mathrm{p} K_{b}$ of a weak base $(\mathrm{BOH})$ are 3.2 and 3.4 , respectively. The $\mathrm{pH}$ of their salt $(A B)$ solution is
(2017 Main)
(a) 7.2
(b) 6.9
(c) 7.0
(d) 1.0
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Solution:
- For a salt of weak acid and weak base,
$$ \mathrm{pH}=7+\frac{1}{2} \mathrm{pK}{a}-\frac{1}{2} \mathrm{pK}{b} $$
Given, $\mathrm{p} K_{\mathrm{a}}(\mathrm{H} A)=3.2, \mathrm{p} K_{a}(B \mathrm{OH})=3.4$
$$ \begin{aligned} \therefore \quad \mathrm{pH} & =7+\frac{1}{2}(3.2)-\frac{1}{2}(3.4) \ & =7+1.6-1.7=6.9 \end{aligned} $$
