From the following enthalpies of combustion, deduce which of the quoted expressions gives the heat of formation of CH4 at a fixed temperature.
C + O2(g) → CO2(g) ; ΔH = c
H2(g) + 1 ⁄ 2 O2(g) → H2O(l) ; ΔH = h
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ;
ΔH = m :
(a) c + 2h − m
(b) −1 ⁄ 2c—h + 1 ⁄ 2m
(c) c + h − m
(d) −c − h + m