The Clausius-Clapeyron Equation: $\ln \left( \frac{P_2}{P_1} \right) = \frac{\Delta H_{vap}}{R} \left( \frac{1}{T_1} - \frac{1}{T_2} \right)$ Where $P_1, P_2$ are vapour pressures at temperatures $T_1, T_2$ (in Kelvin), and $\Delta H_{vap}$ is the Enthalpy of Vaporization.