Question

What is the conjugate base of $HN{O_2}$?

Answer 1

The term 'conjugate base' is usually used in conjunction with the term 'conjugate acid' in the chemical world, and it comes from the Brønsted-Lowry theory of Acids and Bases.
The Brønsted-Acid is a proton donor, while the Brønsted-Base is a proton acceptor. These definitions mean that a chemical reaction is needed to distinguish between the 'donor' and 'acceptor' substances. The root of the application can be traced back to the ' Brønsted -Lowry' principle of acids and bases.

Complete step by step answer:
By extracting a proton $\left( {{H^ + }} \right)$from any Brønsted-Lowry acid, the conjugate base can be identified (every Brønsted-Lowry acid has a conjugate base, and vice versa).
Let's look at the chemical reaction where nitrous acid reacts with water:
$HN{O_2}\left( {aq} \right) + {H_2}O\left( l \right) \rightleftharpoons NO_2^ - \left( {aq} \right) + {H_3}{O^ + }\left( {aq} \right)$
Here, to form $NO_2^ -$ and the hydronium ion, ${H_3}{O^ + }$ the Brønsted-Lowry acid $HN{O_2}$, has donated a proton to ${H_2}O$. This is the forward reaction and in the reverse reaction, $NO_2^ -$ is now the Brønsted-Lowry base (conjugate of $HN{O_2}$) because it accepts a proton from hydronium (conjugate acid of ${H_2}O$) to form nitrous acid again.
So, the conjugate base of $HN{O_2}$ is $NO_2^ -$

Note:
In an aqueous solution, a monoprotic acid donates either one proton or hydrogen atom per molecule. Monoprotic acids include $HN{O_2}$, also known as nitrous acid, is a weak acid that only occurs in the form of nitrite salts in solution $\left( {NO_2^ - } \right)$. Remember that the presence of one hydrogen distinguishes a Conjugate Base from an Acid of the Conjugate Base. That is, adding one hydrogen to any anion produces the conjugate base's acid.
Generally:
$Anio{n^ - } + {H^ + } \Rightarrow H - Anion$
$Anio{n^ - } \Rightarrow$Conjugate Base of the acid $H - Anion$