$\Delta_{\text {vap }} \mathrm{H}^{\ominus}$ for water is $+40.79 \mathrm{~kJ} \mathrm{~mol}^{-1}$ at 1 bar and $100^{\circ} \mathrm{C}$. Change in internal energy for this vapourisation under same condition is ________ $\mathrm{kJ} \mathrm{~mol}^{-1}$. (Integer answer) (Given $\mathrm{R}=8.3 \mathrm{~JK}^{-1} \mathrm{~mol}^{-1}$)