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At 300 K, the standard enthalpies of for...

At 300 K, the standard enthalpies of formation of `C_(6)H_(5)COOH (s), CO_(2)(g)` & `H_(2)O (l)` are , `-408, -393` & `-286" kJ mol"^(-1)` respectively.
Calculate the heat of combustion of benzoic acid at :
(i) constant pressure
(ii) constant volume.

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The correct Answer is:
(i) -3201 kJ/mol (ii) -3199.75 kJ/mol
`C_(6)H_(5)COOH+15/2 O_(2) rarr 7CO_(2)+3H_(2)O(l)`
`DeltaH=(7xx-393)+3(-286)-(408)=-3201`
`DeltaU=DeltaH-Deltan_(g) RT`
`Deltan_(g)=-1/2`
`DeltaU=-3199.75 kJ mol^(-1)`
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