Question

Calculate ΔH∘f (in kilojoules per mole) for benzene, C6H6, from the following data:
2C6H6(l)+15O2(g)→12CO2(g)+6H2O(l) ΔH∘ = -6534kJ
ΔH∘f (CO2) = -393.5kJ/mol
ΔH∘f (H2O) = - 285.8kJ/mol

Answer 1

ΔH for the reaction = [sum of heat of formation for products] - [sum of heat of formation for reactants]
   
-6534 = [(12)(-393.5) + (6)(-285.8)] - (2X)

-6534 = [(-4722) + (-1714.8)] - 2X

-6534 = -6436.8 - 2X

-6534 + 6436.8 = -2X

-97.2 = -2X
 
then, divide each side by two to cancel out the numerical coefficient.

x = +48.6 kJ, which is the heat of formation for benzene.