Enthalpy of combustion of carbon, hydrogen and C3H8 are x1,x2 and x3 mol−1 respectively. Estimate the enthalpy of formation of C3H8
3x1−4x2−x3
C3H8(g)+5O2→3CO2(g)+4H2O(l),ΔH=x3
ΔHr=x3=3ΔHf(CO2)+4ΔHf(H2O)
−Δf(C3H8)−5ΔHf(O2)
=3x1+4x2−ΔHf(C3H8)−5×0
or ΔHf(C3H8)=3x1+4x2−x3