Using the heat of combustion of octane C8H18 (l) -5470.5 and heat of formation table determine the enthalpy formation value for octane.
First of all, let's balance the equation:
C8H18(l) + 25/2 O2 ------> 8 CO2 + 9 H2O -5470.5 kJ
Now, we need the other reactions that we can look up in the standard heat of formation table as you mentioned.
C(s) + O2(g) ----> CO2(g) -394 kJ
But to balance this with the first equation, we need to multiply it by 8. Therefore,
8 C(s) + 8 O2(g) ...
This solution uses the heat of combustion of octane to determine its enthalpy formation value.