The heat of combustion of `CH_(4)`,C(graphite) and `H_(2)(g)` are respectively 20 kcal and -40 kcal , -10 kcal. The heat of formation of `CH_(4)` is

Options

A. 40kcal
B. `+kcal`
C. 80 kcal
D. `-70 kcal`

Answer

D

Explanation

The following equations are obtained the given data
(i) `CH_(4)+2O_(2)toCO_(2)+2H_(2)O,DeltaH=20kcal`
(ii) `C+O_(2)toCO_(2)," "Delta=-40kcal`
(iii) `2H_(2)+O_(2)to2H_(2)O,""""DeltaH=-10kcal`
Note
The value of heat of combustion is always negative
`therefore` Heat of combustion of `CH_(4)`must be -`20kcal`
Reverse equation (ii) and add all the equations
heat of formation of` CH_(4)=(-20)+(-40)+(-10)kcal`
`=-70kcal`