Enthalpies of combustion of carbon, Hydrogen, and benzene are -94kcal/mole, - 68kcal/mole and -780kcal/mole respectively. Enthalpy of formation of benzene is
 

$$\begin{gathered} \mathrm{given}\mathrm{C}\left(\mathrm{graphite}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right);\mathrm{\Delta H}=-94\mathrm{kcals}--\left(1\right) \\ {\mathrm{H}}_2\left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right);\mathrm{\Delta H}=-68\mathrm{kcals}---\left(2\right) \\ {\mathrm{C}}_6{\mathrm{H}}_6\left(\mathrm{l}\right)+\frac{15}{2}{\mathrm{O}}_2\left(\mathrm{g}\right)\to 6{\mathrm{CO}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right);\mathrm{\Delta H}=-780\mathrm{kcals}---\left(3\right) \end{gathered}$$

                 Required:

$$\begin{gathered} 6\mathrm{C}\left(\mathrm{graphite}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\to {\mathrm{C}}_6{\mathrm{H}}_6\left(\mathrm{l}\right);\mathrm{\Delta H}=? \\ 6\times \mathrm{eq}\left(1\right)+3\times \mathrm{eq}\left(2\right)-Eq\left(3\right) \\ {\mathrm{\Delta }}^0{\mathrm{H}}_{\mathrm{f}}\left({\mathrm{C}}_6{\mathrm{H}}_6\right)=6\left(-94\right)+3\left(-68\right)-\left(-780\right) \\ =\left[-564-204+780\right]\mathrm{kJ} \\ \begin{array}{l}=\left[-768+780\right]\mathrm{kJ}\\ =+12\mathrm{kJ}\end{array} \end{gathered}$$