`H_(2)(g)+1//2O_(2)(g)=H_(2)O(l) , Delta H_(298 K)=-68.32` Kcal. Heat of vapourisation of water at 1 atm and `25^(@)C` is 10.52 Kcal. The standard heat of formation (in Kcal) of 1 mole of water vapour at `25^(@)C` is

Options

A. `10.52`
B. `-78.84`
C. `+57.80`
D. `-57.80`

Answer

D

Explanation

`H_(2(g))+(1)/(2)O_(2(g))rarr H_(2)O_((g)) , Delta H_(f) = ?`
`H_(2(g))+(1)/(2)O_(2(g))rarr H_(2)O_((l)) , Delta H=-68.32 KCal`
`H_(2)O_((l))rarr H_(2)O_((g)) , Delta H_("Vap")=10.52 KCal`
eq(i) = eq(ii) + eq(iii)
`Delta H_(f)=(-68.32)+(10.52)`
`Delta H_(f)=-57.80 KCal mol^(-1)`