Standard enthalpy of vapourisation `Delta_("vap")H^( )` for water at `100^@C` is `40.66 kJ mol^( 1)`. The internal energy of vapourisation of water at `100^@C` (in kJ `mol^( 1)`) is (Assume water vapour to behave like Calculated and Experimental Enthalpies vap H and vap H exp of Download Table 24 19. (a) Use Trouton's rule, Delta Hvap^circ approx (88 Entropy of vaporisation is given by the formula, \Delta S_{\text {vap. }}..