For water $\Delta_{\text {vap }} \mathrm{H}=41 \mathrm{~kJ} \mathrm{~mol}^{-1}$ at 373 K and 1 bar pressure. Assuming that water vapour is an ideal gas that occupies a much larger volume than liquid water, the internal energy change during evaporation of water is $\_\_\_\_$ kJ mol-1 [Use: $\mathrm{R}=8.3 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}$ ]