AT `300K`, the standard enthalpies of formation of `C_(6)H_(5)COOH(s), CO_(2)(g)`, and `H_(2)O(l)` are `-408, 393`, and `-286 kJ mol^(-1)`, respectively. Calculate the heat of combustion of benzoic acid at (a) constant pressure and (b) constant volume. `(R = 8.31 J mol^(-1)K^(-1))`

Options

A. `C_(2)H_(2)`
B. `C_(2)H_(6)`
C. Both (A) & (B)
D. None of these

Answer

A

Explanation

Calorific value `=(DeltaH)/("mol.wt")`
for ethane `(C_(2)H_(6))= -(341)/(30)= -11.37`
for ethyne `(C_(2)H_(2))= -(310)/(26)= -11.9`
So, better calorific value is better fuel hence, `C_(2)H_(2)`