On complete combustion of 2 gm methane 26575 cals of heat is generated. The enthalpy of formation of methane will be (Given entha lpy of formation of `CO_2 and H_(2)O` are - 97000 cals and -68000 cals respectively):

Options

A. `+20400` cals
B. `+20600` cals
C. `-20400` cals
D. `-2000` cals

Answer

C

Explanation

`(Delta H_("comb."))_(CH_(4))=(-26575)/(2)xx16=-212600 Cal mol^(-1)`
`CH_(4)+2O_(2)rarr CO_(2)+2H_(2)O , Delta H_("comb.")=-212600 Cal mol^(-1)`
`Delta H=Sigma (Delta H_(f))_(P)-Sigma (Delta H_(f))_(R )-212600 =[(-97000)+2xx(-68000)]=[Delta H_(f)(CH_(4))+0]`
`Delta H_(f)(CH_(4))=-233000+212600`
`therefore Delta H_(f)(CH_(4))=-20400 Cal`