The correct option is D −20 kcal
Given, C(s)+O2(g)→CO2(g) ΔH=−94.2 kcal....(1)
H2(g)+12O2(g)→H2O(l) ΔH=−68.3 kcal....(2)
CH4(g)+2O2(g)→CO2(g)+2H2O(l) ΔH=−210.8 kcal....(3)
we need C(s)+2H2(g)→CH4(g)
multiply equation 2 by 2, we get
2H2(g)+O2(g)→2H2O(l) ΔH=+2×(−68.3) kcal............4
reversing the equation 3, we get
CO2(g)+2H2O(l)→CH4(g)+2O2(g) ΔH=(+210.8) kcal.........5
adding eqn 1, 4 and 5 to get C(s)+2H2(g)→CH4(g)
so, heat of formation of methane =(−94.2+2×(−68.3)+210.8) kcal
=−20 kcal