Experiments show that the vapour pressure #P#, enthalpy of vaporization, #ΔH_"vap"#, and temperature #T# are related by the equation

#lnP = "constant" – (ΔH_"vap")/"RT"#

where #R# is the ideal gas constant. This equation is the Clausius- Clapeyron equation.

If #P_1# and #P_2# are the vapour pressures at two temperatures #T_1# and #T_2#, the equation takes the form:

#ln(P_2/(P_1)) = (ΔH_"vap")/R(1/T_1 – 1/T_2)#

The Clausius-Clapeyron equation allows us to estimate the vapour pressure at another temperature, if we know the enthalpy of vaporization and the vapor pressure at some temperature.

Example

Ethanol has a heat of vaporization of 38.56 kJ/mol and a normal boiling point of 78.4 °C. What is the vapor pressure of ethanol at 50.0 °C?