there are three oxoacids of nitrogen.

1. Nitric acid(HNO₃)

2. Nitrous acid (HNO₂)

3. Hyponitrous acid (H₂N₂O₁)

+ 3 oxidation is shown by HNO₂, Therefore it undergoes disproportion reaction,

To calculate oxidation state, consider the oxidation state of N is x.

3HNO₂ HNO₃ + 2NO +H₂O

Oxidation state of nitrogen in HNO₂ is +3

X + (+1) +2*(-2) = 0, x= +3

Oxidation state of nitrogen in HNO₃ is +5

X + (+1) + 3*(-2) = 0, x= 5

Oxidation state of nitrogen in NO is +2

X + (-2) = 0, x =2

Therefore +3 oxidation state changes to +5 and +2 oxidation states