In which of the following compounds is the oxidation number of nitrogen equal to + 3?

The oxidation number of an element is the hypothetical charge it would have if all its bonds were purely ionic. In other words, it is a measure of the degree of oxidation of an atom in a molecule. To determine the oxidation number of nitrogen in each compound, we need to know the electronegativity of the other elements involved in the bond. In NO₂, the electronegativity of oxygen is higher than nitrogen, so nitrogen is assigned an oxidation number of +4. In N₂O, the oxidation number of nitrogen is +1. In NO, nitrogen is assigned an oxidation number of +2. In HNO₂, nitrogen is assigned an oxidation number of +3. In HNO₃, nitrogen is assigned an oxidation number of +5. Therefore, the only compound in which the oxidation number of nitrogen is equal to +3 is HNO₂.