The formation of nitric acid from nitrogen, hydrogen, and oxygen can be written as,
N₂ + H₂ + 3O₂ --> 2HNO₃
The net enthalpy of formation of nitric acid is calculated by,
Hrxn = Hproduct - Hreactant
Since all the reactants are in their elemental forms, the simplified equation would be,
Hrxn = Hproduct
Substituting,
Hrxn = (-186.81 kJ/mol)(2 mols)
Answer: -372.42 kJ