Chemistry I / Physical Equilibria and Phase Diagrams
Practice question · Numerical answer

A liquid boils at 350 K with an enthalpy of vaporisation of 35 kJ/mol. Compute its entropy of vaporisation in J/mol/K.

Hints
  1. At the boiling point the liquid and vapour are in equilibrium, so dG is zero.
  2. Compute 35000/350.
Show the answer

100 (answers within ±1 count)

Why

ΔSvap=ΔHvap/Tb=35,000/350=100\Delta S_{vap} = \Delta H_{vap}/T_b = 35{,}000/350 = 100 J/mol/K. Most liquids cluster near 88 J/mol/K (Trouton’s rule); a markedly higher value signals strong hydrogen bonding in the liquid.

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