The heats of formation of `CO_(2(g)) , H_2O_((l)) and CH_(4(g))` are respectively `–394 kJ mol^(-1), -286 kJ mol^(-1) and -76 kJ mol^(-1)`. Calculate the heat of combustion of methane. 

Explanation

`C_("graphite")  + O_(2(g)) to CO_(2(g)) , DeltaH_1 = -394 kJ`
  `H_(2(s)) + 1/2O_(2(s)) to H_2O_((l)) , DeltaH_2 = -286 kJ`
The requird equation is ,
`CH_(4(g)) + 2O_(2(g)) to CO_(2(g)) + 2H_2O_((l))  , DeltaH = ?`
`DeltaH = DeltaH_1 + 2DeltaH_2 - DeltaH_3 = -394 + 2(-286)-(-76)`
The heat of combustion of methane `= - 890 kJ mol^(-1)`.