Question

Select the correct option(s) from the following:

• A. $pH$ of ${10}^{-8}MHCl$ is 8 as $pH=-log\left[H^{+}\right]$
• B. $H_{2}P{O}_4^{-}$ can act as both Bronsted acid and Bronsted base and is an example of an amphiprotic ion
• C. $H_{2}P{O}_2^{-}$ can act as both Bronsted acid and Bronsted base and is an example of an amphiprotic ion
• D. When a weak base $BOH$ $K_b={10}^{-5}$ is titrated with a strong acid HCl. At $\frac{4}{5}th$ of the equivalence point the pH of the solution will be 8.4

Answer 1

Answer: (B), (D)

The correct options are
B $H_{2}P{O}_4^{-}$ can act as both Bronsted acid and Bronsted base and is an example of an amphiprotic ion
D When a weak base $BOH$ $K_b={10}^{-5}$ is titrated with a strong acid HCl. At $\frac{4}{5}th$ of the equivalence point the pH of the solution will be 8.4

(a) The pH of $HCl$ solution can not be 8. We need to consider the dissociation of water here, and when we do so, we get a pH slightly lower than 7.
(b) $H_{2}P{O}_4^{-}$ can accept as well as lose $H+$ ions. So, it can act as both: a Bronsted acid and a base and hence, is amphoprotic.
(c) $H_{2}P{O}_2^{-}$ cannot donate any more $H^{+}$ ions.
(d) At $\frac{4}{5}$th equivalence point [salt]=$\frac{4}{5}$
and [base]=$\frac{1}{5}$ and a basic buffer will be formed.
$pOH=p{K}_b+log\frac{\left[salt\right]}{\left[base\right]}$
$pOH=5.6$
$pH=14-5.6=8.4$