The vaporization curves of most liquids have similar shape. The vapour pressure steadily increase as the temperature increases. A good approach is to find a mathematical model for the pressure increase as a function of temperature. Experiments showed that the pressure P, enthalpy of vaporization, DH_vap, and temperature T are related,P = A exp (- DH_vap / R T)where R (= 8.3145 J mol^-1 K^-1) and A are the gas constant and unknown constant. This is known as the Clausius- Clapeyron equation. If P₁ and P₂ are the pressures at two temperatures T₁ and T₂, the equation has the form:

     P1    DHvap    1     1 ln (---) = ----  (--- - ---)      P2     R      T2    T1 

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