hi,

Reaction of combustion in benzene.

C6H6 + 7½O2 (g) -----> 6CO2 (g) + 3H2O (l)

In the above reaction O2 and CO2 are the only gases.

g = no. of moles of products – no. of moles of reactants

= 6 - 7½ = - 1 ½ = -3/2

Given values U = 3263.9 kJ/mol; T = 298 K;

As we know,

H =U +ng RT R = 8.314

As this is an exothermic reaction,H is negative.

H = (- 3263.9 kJ/mol) + (-3/2) (298 K)

= (- 3263.9 kJ/mol) + (-3/2) (2.477)

= (- 3263.9 kJ/mol) – 3.716 kJ/mol

H = - 3267.6 kJ/mol