Question

The conjugate base of hydrazoic acid is:
A.${N^{ - 3}}$
B.${N_3}^ -$
C.${N_2}^ -$
D.$H{N_3}^ -$

Answer 1

A conjugate base is a chemical substance which is formed when an acid loses a proton. According to Bronsted–Lowry acid-base theory acids are substances which are proton donors. So the product after losing a proton is referred to as the conjugate base.

Complete step by step answer:
A conjugate base is formed from acid by removal of protons. So addition of proton to a conjugate base will result in an acid. As hydrazoic acid is an acid so it will be produced by adding a proton to its conjugate base.
The molecular formula of hydrazoic acid is $H{N_3}$. After adding protons to each of the conjugate bases, the formula of the compound obtained matches with the desired formula of hydrazoic acid. Then that base will be considered as the corresponding conjugate base of hydrazoic acid.
Let us consider the addition of protons to the given conjugate base one by one.
${N^{ - 3}} + 3{H^ + } \to N{H_3}$
The product of adding protons to ${N^{ - 3}}$ gives $N{H_3}$ which is ammonia and not hydrazoic acid.
${N_3}^ - + {H^ + } \to H{N_3}$
The product obtained by addition of proton to ${N_3}^ -$ gives $H{N_3}$ which is hydrazoic acid.
${N_2}^ - + {H^ + } \to H{N_2}$
The product of adding protons to ${N_2}^ -$ gives $H{N_2}$ which is not hydrazoic acid.
$H{N_3}^ - + {H^ + } \to {H_2}{N_3}$
The product of adding protons to $H{N_3}^ -$ gives ${H_2}{N_3}$ which is not hydrazoic acid.
Therefore option B is correct i.e.${N_3}^ -$.
Thus the dissociation of hydrazoic acid can be written as:
$H{N_3} \to {H^ + } + {N_3}^ -$

Note: In a similar way a conjugate acid is formed from a base by addition of a proton. For example ammonia is a base. When a proton is added to ammonia, ammonium ion is generated which is the conjugate acid of ammonia base. They are also known as conjugate acid-base pairs.