Posted by Ellie on .
Given the following heats of combustion.
CH3OH(l) + 3/2 O2(g) CO2(g) + 2 H2O(l) ΔH°rxn = 726.4 kJ
C(graphite) + O2(g) CO2(g) ΔH°rxn = 393.5 kJ
H2(g) + 1/2 O2(g) H2O(l) ΔH°rxn = 285.8 kJ
Calculate the enthalpy of formation of methanol (CH3OH) from its elements.
C(graphite) + 2 H2(g) + 1/2 O2(g) CH3OH(l)

Chemistry 
DrBob222,
CH3OH(l) + 3/2 O2(g) CO2(g) + 2 H2O(l) ΔH°rxn = 726.4 kJ
C(graphite) + O2(g) CO2(g) ΔH°rxn = 393.5 kJ
H2(g) + 1/2 O2(g) H2O(l) ΔH°rxn = 285.8 kJ
Reverse eqn 1, add in eqn 2, add in 2x the reverse of eqn 3. That will give you the final equation you want. When you reverse an eqn you must change the sign of delta H. If you multiply an equation, multiply delta H by the same number.