What is the heat of formation for octane?

Question

Taking into account the heat of combustion for octane (and its equation):

C8H18(l) + 25/2 O2(g) --> 8CO2(g) + 9H2O(g) ΔH= -5472

And, the heat of combustion for the following reactions:

H2(g) + 1/2 O2(g) --> H2O(g) ΔH= -242 kJ

8C(s) + 9H2(g) --> C8H18 ΔH= ?

C(s) + O2(g) --> CO2(g) ΔH= -394 kJ

Thanks in advance for your help!

Answer 1

-249.95 kJ/mol