Question

Select incorrect statement(s) among the following.

• A. Phenolphthalein is suitable indicator for the titration of HCl (aq) with $N{H}_{4}OH\left(aq\right)$
• B. An acid-base indicator in a buffer solution of $pH=p{K}_{ln}+1$ is ionized to the extent of $\frac{1000}{11}$%
• C. In the titration of a monoacidic weak base with a strong acid, the pH at the equivalent point is always calculated by $pH=\frac{1}{2}[p{K}_w-p{H}_b-logC]$
• D. When $N{a}_{3}P{O}_4\left(aq\right)$ is titrated with HCl(aq), the pH of solution at second equivalent point is calculated by $\frac{1}{2}[p{K}_{a_1}+p{K}_{a_2}]$

Answer 1

Answer: (A), (C)

Phenolphthalein is suitable indicator for the titration of HCl (aq) with $N{H}_{4}OH\left(aq\right)$
In the titration of a monoacidic weak base with a strong acid, the pH at the equivalent point is always calculated by $pH=\frac{1}{2}[p{K}_w-p{H}_b-logC]$

$A.$ Phenolphthalein gives a colour change when the $pH$ range from $8.3$ to $10$ i.e., in slightly basic solution. Titration of weak base $N{H}_{4}OH$ with strong acid $HCl$ will finally make the solution acidic, and phenolpthalein will not give colour change or denote the end point correctly.

$C.$ At equivalent point, all the weak base reacts with strong acid and the salt of this base with the strong acid is formed.

For a salt of weak base and strong acid.

$pH=7-\frac{1}{2}[p{K}_b+\log C]=\frac{1}{2}[p{K}_w-p{K}_b-\log C]$