To determine the oxidation number of nitrogen in nitric acid, we must consider the charges of the other atoms in the molecule and the overall charge of the molecule itself. In HNO3, we have one hydrogen atom with an oxidation number of +1, and three oxygen atoms with an oxidation number of -2 each. The overall charge of nitric acid is 0, as it is a neutral molecule.

Let x represent the oxidation number of nitrogen. Using the formula for calculating the overall charge of a molecule:

(+1) + x + (3 × -2) = 0

+1 + x - 6 = 0

x + (-5) = 0

x = +5

Therefore, the oxidation number of nitrogen in nitric acid is +5.