Assuming that water vapour is an ideal gas, the internal energy change (∆U) when 1 mol of water is vapourised at 1 bar pressure and 100∘ C, (given : molar enthalpy of vapourisation of water at 1 bar and 373 K = 41 kJ mol−1 and R = 8.3 J mol−1 K−1 ) will be
4.100 kJ mol−1
△H=41kJ mol−1 =41000Jmol−1
T=100∘C=273+100=373K
n = 1
△U=△H−△nRT=41000−(2×8.314×373)
=37898.88J mol−1=37.9 kJ mol−1