If we have :

Question

H2(g) + 1/2 O2(g) -> H2O(l) dHf= -285.5 Kj
N2O5(g) + H2O(l) -> 2HNO3 dHf = -76.6 Kj
1/2 N2 + 3/2 O2 + 1/2 H2 -> HNO3 dHf = - 174.1 Kj

then , what is the dHf of
2N2 + 5O2 -> 2N2O5

Answer 1

Reverse equation 3 and add to the reverse of equation 2 and add twice equation 3. When you get the final equation it will be 1/2 of what you want; just multiply that by 2.
When reversing equations change the sign of dH. When multiplying equations, multiply the dH value by the multiplier.

Answer 2

I'm confused , can you do the solution please ?