Trouton's Rule
The empirical observation that the entropy of vaporisation of many liquids is close to 88 J/(mol·K) at their boiling point.
Formula / equation
ΔS_vap = ΔH_vap / T_b ≈ 88
Unit
J/(mol·K)
In depth
The regularity arises because the entropy gained on going from liquid to gas is dominated by the change of state itself rather than by the substance's identity. Liquids with strong hydrogen bonding, which are unusually ordered, show clear positive deviations from the rule.
Examples in the real world
Benzene gives 87 J/(mol·K), obeying the rule, while water gives 109 because of its hydrogen-bonded network.