Question

Which of the following is the strongest conjugate base?
(A) $C{l^ - }$
(B) $C{H_3}CO{O^ - }$
(C) $SO_4^{ - 2}$
(D) $NO_2^ - $

Answer 1

Hint: An acid loses a proton to form an anion known as its conjugate base. A strong acid will form a weak conjugate base and a weak acid will form a strong conjugate base. So, in this question first evaluate the acid associated with each conjugate base and then check for the weakest acid for its conjugate base to be strongest.

Complete step by step answer:
-First of all let us see what a conjugate base is.
A conjugate base is a substance that forms from an acid and has the ability to accept protons. It is basically a base member, ${X^ - }$, of some pair of compounds which can transform into each other by gaining or losing a proton. The conjugate base can absorb or gain a proton in a chemical reaction, while the conjugate acid donates the proton or hydrogen in the reaction.
In an acid-base reaction, the chemical reaction is:
$Acid + Base \rightleftharpoons Conj.Base + Conj.Acid$
In short we can say that an acid loses a proton to form its conjugate base.
-Also we should remember that a weak acid forms strong conjugate base and strong acid forms a weak conjugate base.
-In the question we need to find out which of the options is the strongest conjugate base. So, we will first check which acids will form these conjugate bases and find out the weakest acid among all since the weakest acid will form the strongest base.
To find out the acid we will add a proton to each of the conjugate bases.
For (A) $C{l^ - }$: This conjugate base is formed from acid: Hydrochloric acid (HCl). It is a strong acid and so this cannot form a strong conjugate base.
For (B) $C{H_3}CO{O^ - }$: This conjugate base is formed from acetic acid ($C{H_3}COOH$). It is a weak acid and so it can form a strong conjugate base.
For (C) $SO_4^{ - 2}$: It is formed from acid: Sulphuric acid (${H_2}S{O_4}$). It is also a very strong acid and so cannot form a strong conjugate base.
For (D) $NO_2^ - $: It is formed from acid: Nitrous acid ($HN{O_2}$). It is a weak monoprotic acid and so can form a strong conjugate base.
-In the above discussion we saw that there are two weak acids which form their conjugate base, they are: acetic acid ($C{H_3}COOH$) and nitrous acid ($HN{O_2}$). Among both of them acetic acid is the weakest acid and so it will form the strongest conjugate base.
-Hence $C{H_3}CO{O^ - }$ will be the strongest conjugate base because it has been formed from the weakest acid ($C{H_3}COOH$).

-So, the correct option will be: (B) $C{H_3}CO{O^ - }$

Note: The concept of conjugate acids and bases belongs to the Bronsted-Lowry theory of acids and bases. According to this theory any species that has the ability to donate a proton in a reaction is a conjugate acid and the species that has the ability to accept a proton is the conjugate base. Also the conjugate base exists as an anion while the conjugate acid exists as a cation.