Question

Nitrogen oxides undergo many interesting reactions in the environment and in industry. Given the following information, calculate H for the overall equation: 2 NO2(g) + 1/2 O2(g) = N2O5(s).
2 NO(g) + 3/2 O2(g) = N2O5(s), H = -223.7 kJ
NO(g) + 1/2 O2(g) = NO2(g), H = -57.1 kJ

Answer 1

Multiply equation 2 by 2 and reverse it.
Add in equation 1.
That will give you what you want.

Answer 2

-109.5