Answer
30.1 kJ/mol
Work Step by Step
Recall that $\Delta S_{vap}=\frac{\Delta H_{vap}}{T_{b}}$.
$\implies \Delta H_{vap}=\Delta S_{vap}\times T_{b}$
Given that $T_{b}=80.7^{\circ}C=(80.7+273.15)K=353.85\,K$
For cyclohexane, $\Delta S_{vap}=85.1\,J\,K^{-1}mol^{-1}$
Then, $\Delta H_{vap}=(85.1\,J\,K^{-1}mol^{-1})(353.85\,K)$
$=30100\,J\,mol^{-1}=30.1\,kJ/mol$
