Trouton's rule

Trouton's rule, proposed by the physicist Frederick Thomas Trouton in 1884, is a principle in thermodynamics stating that the molar entropy of vaporization has almost the same value, about 85 to 88 joules per kelvin per mole, for many different liquids at their normal boiling points. The entropy of vaporization is calculated as the ratio of a liquid's enthalpy of vaporization to its boiling temperature, and the rule is often expressed using the gas constant R as approximately 10.5R. Chemists commonly use the rule to estimate a liquid's enthalpy of vaporization once its boiling point is known, although liquids with strong molecular interactions, such as hydrogen bonding in water and ethanol, or those that form ordered structures in the gas phase, such as formic acid, deviate significantly from the predicted value.