The heat of combustion of CH₄ (g) , C (graphite) and H₂ (g) are respectively -20 K.cal, - 40 K.cal and -10 K.cal . The heat of formation of CH₄ is

 

C(s) + 2H₂ (g) ® CH₄ (g)

CH₄ (g) + 2 O₂ ® CO₂ + 2 H₂O + 20 k.Cal 

C(s) + O₂ $\to$CO₂ + 40 k.Cal

2 H₂ + O₂ ® H₂O + 20 k.Cal

Total energy released when CO₂ and H₂O formation

= 40 + 20 = 60 k.Cal