Chemical and Ionic Equilibrium 2 Question 53
53. $0.1 \mathrm{M}$ of $\mathrm{H} A$ is titrated with $0.1 \mathrm{M} \mathrm{NaOH}$, calculate the $\mathrm{pH}$ at end point. Given, $K_{a}(\mathrm{H} A)=5 \times 10^{-6}$ and $\alpha«1$.
(2004)
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Solution:
- At the end-point, $\left[A^{-}\right]=0.05$
$$ \begin{aligned} K_{b}\left(A^{-}\right) & =K_{w} / K_{a}=2 \times 10^{-9} \ {\left[\mathrm{OH}^{-}\right] } & =\sqrt{K_{b} C}=\sqrt{2 \times 10^{-9} \times 0.05}=10^{-5} \ \mathrm{pOH} & =5 \quad \text { and } \quad \mathrm{pH}=9 \end{aligned} $$
