Question

The conjugate base of ${H_2}P{O_4}^ -$ is:
(A) $P{O_4}^{3 - }$
(B) $HP{O_4}^{2 - }$
(C) ${H_3}P{O_4}$
(D) ${P_2}{O_5}$

Answer 1

Hint : Conjugate acid and conjugate base are always on the right side of the reaction as products. There is only a difference in addition and removal of proton in conjugate acid-base pairs followed by charge related to proton.

Complete step by step answer
Let us first see what conjugate base is. Actually conjugate base comes into picture when we talk about conjugate acid-base pairs. Conjugate acid consists of one extra hydrogen atom and a positive charge as compared to the base from which it is formed and conjugate base consists of one less hydrogen atom and an extra negative charge as compared to the acid from which it is formed.
Let us understand it from a simple chemical reaction,
$HCl + N{H_3} \rightleftharpoons N{H_4}^ + + C{l^ - }$
Here we have hydrochloric acid and ammonia as base. Now in this reversible reaction, hydrochloric acid loses one hydrogen atom and the remaining chlorine gains a negative charge so $C{l^ - }$ is the conjugate base of hydrochloric acid. Ammonia which is the base gains a hydrogen atom and acquires a positive charge so $N{H_4}^ +$ is the conjugate acid of ammonia.
So in this question we are asked for the conjugate base of ${H_2}P{O_4}^ -$ , hence we will have to remove one hydrogen atom and also put a negative charge. Therefore the resulting compound will be $HP{O_4}^{2 - }$ .
So option B is the correct answer.

Note
The chemical meaning of the word conjugate is to join together. The concept of conjugate acid-base pair was given in Bronsted-Lowry acid-base theory. The proton mentioned in conjugate acid-base theory is nothing but ${H^ + }$ ion. Weak acids have stronger conjugate base and strong acids have weak conjugate base.