Question

For a particular isomer of C8H18, the following reaction produces 5093.7 KJ of heat per mole of C8H18(g) consumed, under standered conditions. What is the standard enthalpy of formation of this isomer of C8H18(g)?

Answer 1

delta Hf = dHf 
dHf rxn = (n*dHf products) - (n*dHf reactants). 
Substitute 5113.3 kJ for dHf rxn and solve for the only unknown in the equation which is dHf C8H18. 
-5133.3-[(8x-393.5)+9(-241.8)] = 190.9 

Since it is an enthalpy of formation, the value has to be negative, 
thus the correct answer is -190.9