Question

The heats of formation of $C_6{H}_6\left(l\right),H_{2}O\left(l\right),C{O}_2\left(g\right)$ are $11.70,-68.4$ and $-94.0$ $kcal$ respectively. Calculate the heat of combustion of benzene($l$).

• A. −780.9 kcal−780.9kcal
• B. −680.9 kcal−780.9kcal
• C. 580.9 kcal−780.9kcal
• D. 480.9 kcal−780.9kcal

Answer 1

The correct option is A −780.9 kcal−780.9kcal

Heat of combustion of benzene = 3$\times$ heat of formation of water + 6$\times$ formation $C{O}_2$ - heat of foemation of benzene

Heat of combustion of benzene = $3\ast (-68.4)+6\ast (-94)-\left(11.70\right)$=$-780kcal$