$7C_{(s)} + 3H_{2(g)} + O_{2(g)} \rightarrow C_6H_5COOH_{(s)}, \Delta H^o = -408 \,KJ ………..(1)$

$C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)} ; \Delta H^o = -393 \,KJ …….(2)$

$H_{2(g)} + {1\over 2} O_{2(g)} \rightarrow H_2O_{(l)} \Delta H^o = -286\, KJ ……….(3)$

$C_6H_5COOH_{(s)}$ + ${15\over 2}O_{2(g)} $ $ \rightarrow 7CO_{2(g)} $+ $ {2H_2O}$ $_{(l)} $  $\Delta H = ? ………..(4)$

By, $(2) \times 7 + (3) \times 3 ………(1)$

$\Delta H^o = [- 393 \times 7 - 286 \times 3 + 408]$

$\Delta H^o = - 3201 \,KJ         \Delta H = \Delta U + \Delta nRT$

$-3201 = \Delta U + (-0.5) \times 8.314 \times 10^{-3} \times 300$

$\Delta U = - 3201 + 1.2471 = - 3199.7529 \,kJ$