3 Ways to Calculate Vapor Pressure wikiHow 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 10.10: Enthalpy of Fusion and Enthalpy of Vaporization Chemistry LibreTexts 3. (a) Derive the integrated form of Clausius Clapeyron Entropy of vaporisation is given by the formula, \Delta S_{\text {vap. }}.. Heat of Vaporization from Vapor Pressure (Example)