Question

Write the formula for an oxoacid of nitrogen with N in the oxidation state +3.
(A) ${\text{HN}}{{\text{O}}_{\text{2}}}$
(B) ${{\text{H}}_2}{\text{N}}{{\text{O}}_{\text{2}}}$
(C) ${\text{HN}}{{\text{O}}_3}$
(D) ${\text{HNO}}$

Answer 1

Hint: The term oxoacid or oxyacid is used to refer to an acid which contains oxygen.
- To be more specific, oxoacid is a compound which contains oxygen, contains at least one other element, contains at least one hydrogen bonded to oxygen and can form ions in solution by the loss of one or more protons.
- For example, sulphuric acid or ${{\text{H}}_2}{\text{S}}{{\text{O}}_4}$ is an oxoacid of sulphur.

Complete step by step answer:
Nitrogen is a group 15 element and it forms three oxoacids. These are hyponitrous acid, nitrous acid and nitric acid.
Hyponitrous acid is represented by the molecular formula ${{\text{H}}_2}{{\text{N}}_2}{{\text{O}}_{\text{2}}}$ , nitrous acid is represented by the molecular formula ${\text{HN}}{{\text{O}}_{\text{2}}}$ and nitric acid is represented by the molecular formula ${\text{HN}}{{\text{O}}_3}$ .
Let us first consider nitrous acid or ${\text{HN}}{{\text{O}}_{\text{2}}}$ . The oxidation state of hydrogen is known to be +1 and the oxidation state of oxygen isknown to be -2.
Let the oxidation state of nitrogen be x. Then, sum of all these must be equal to zero as there is no charge on the compound. So,
$
  \left( { + 1} \right) + {\text{x}} + 2 \times \left( { - 2} \right) = 0 \\
   \Rightarrow {\text{x}} = 4 - 1 \\
   \Rightarrow {\text{x}} = 3 \\
 $
Therefore, oxidation state of nitrogen in nitrous acid is +3. So, option A is correct.
Now, let us consider ${{\text{H}}_2}{\text{N}}{{\text{O}}_{\text{2}}}$. Again, the oxidation state of hydrogen is +1 and the oxidation state of oxygen is -2.
Let the oxidation state of nitrogen be x. Then, sum of all these must be equal to zero as there is no charge on the compound. So,
$
  2 \times \left( { + 1} \right) + {\text{x}} + 2 \times \left( { - 2} \right) = 0 \\
   \Rightarrow {\text{x}} = 4 - 2 \\
   \Rightarrow {\text{x}} = 2 \\
 $
So, the oxidation state of nitrogen is +2 and so B is not the correct answer.
Now, let us check nitric acid or ${\text{HN}}{{\text{O}}_3}$. Again, the oxidation state of hydrogen is +1 and the oxidation state of oxygen is -2.
Let the oxidation state of nitrogen be x. Then, sum of all these must be equal to zero as there is no charge on the compound. So,
$
  \left( { + 1} \right) + {\text{x}} + 3 \times \left( { - 2} \right) = 0 \\
   \Rightarrow {\text{x}} = 6 - 1 \\
   \Rightarrow {\text{x}} = 5 \\
 $
So, the oxidation state of nitrogen in nitric acid is +5 and so C is also incorrect.
Lastly, in case of ${\text{HNO}}$ , the oxidation state of nitrogen is +1 as shown below.
$
  \left( { + 1} \right) + {\text{x}} + \left( { - 2} \right) = 0 \\
   \Rightarrow {\text{x}} = 2 - 1 \\
   \Rightarrow {\text{x}} = 1 \\
 $

So, option D is also incorrect.

Note:
- Nitrous acid is a weak monoprotic acid known only in solution or in gas phase or in the form of nitrate salts.
- Nitrous acid is mainly used to make diazonium salts from amines which then act as reagents in azo coupling reactions to produce azo dyes.