Chemical and Ionic Equilibrium 2 Question 39
39. Which of the following statement(s) is (are) correct?
(a) The $\mathrm{pH}$ of $1.0 \times 10^{-8} \mathrm{M}$ solution of $\mathrm{HCl}$ is 8
$(1998,2 M)$
(b) The conjugate base of $\mathrm{H}{2} \mathrm{PO}{4}^{-}$is $\mathrm{HPO}_{4}^{2-}$
(c) Autoprotolysis constant of water increases with temperature
(d) When a solution of a weak monoprotic acid is titrated against a strong base, at half-neutralisation point $\mathrm{pH}=\left(\frac{1}{2}\right) \mathrm{p} K_{a}$
Numerical Value Based Question
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Solution:
- $\mathrm{pH}$ of $10^{-8} \mathrm{M}$ solution will be between 6 and 7 but never 8 . The conjugate base of an acid is formed by removing a proton $\left(\mathrm{H}^{+}\right)$ from acid. Therefore, $\mathrm{HPO}{4}^{2-}$ is a conjugate base of $\mathrm{H}{2} \mathrm{PO}_{4}^{-}$.
$$ \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OH}^{-} $$
$\Delta H>0$
Increasing temperature will increase equilibrium constant of the above endothermic reaction.
At the mid-point of titration $\mathrm{pH}=\mathrm{p} K_{a}$
