Question

The strongest conjugate base is
A) $N{O_3}^{-}$
B) $C{l}^{-}$
C) $S{O_4}^{2-}$
D) $C{H}_{3}CO{O}^{-}$

Answer 1

Hint: An acid always has the tendency to donate protons or $H^{+}$ ions and forms a conjugate base.
A strong acid will dissociate to form a weak conjugate base and the weak acid will dissociate to form a strong conjugate base.

Complete answer:
In the question, we are asked to tell the strongest conjugate base from the options given below.
For that we should have an idea about, what is a conjugate base and how this species i.e. conjugate base is formed. And also we should know how to compare among the strengths of the conjugate bases.
So now let’s have an idea about conjugate base
Conjugate base is a basic species say $Y^{-}$, of any pair of compounds which can gain or can absorb or gain electrons from the chemical reaction and come to be changed into another species of ions.
Conjugate base is formed from an acid, when acid donates protons to the reaction mixture and the leftover part of the acid with the proton is called as the conjugate base.
The reaction of the formation of conjugate base can be written as,
$Acid+Base\rightleftharpoons Conjugateacid+Conjugatebase$
An acid forms the conjugate base and a base forms the conjugate acid.
Another point that we should know is that a strong acid will form a weak conjugate base and a weak acid will form a strong conjugate base.
Now let’s compare the option given and find each acid associated with the given conjugate base and find the weakest acid among them so that we will get the answer as the weak acid will give the strong conjugate base.
Option (A): $N{O_3}^{-}$ is formed by the dissociation of $HN{O}_3$, which is one of the strongest acid known and hence the conjugate base produced will be weak.
Option (B): $C{l}^{-}$ is formed from a strong acid $HCl$ and hence the conjugate base formed also will be a weak conjugate base.
Option (C): $S{O_4}^{2-}$ is also formed from a strong acid $H_{2}S{O}_4$, so $S{O_4}^{2-}$ is a weak conjugate base.
Option (D): $C{H}_{3}CO{O}^{-}$ is formed from the weak acid $C{H}_{3}COOH$, so the conjugate base formed from a weak acid will be strong.

Hence the correct option for this question is option (D).

Note:
A conjugate base always exists as an anion and a conjugate base exists as a cation. The concept of conjugate acid and conjugate base is of Bronsted-Lowry theory of acid and bases.