The Clausius-Clapeyron Equation is used to calculate the vapor pressure at another temperature if the vapor pressure is known at some temperature.
ln(P2/P1) = ΔH/R(1/T1 – 1/T2)
P1 & P2 – mmHg, T1 & T2 – K, R – 8.314J/mol.K , ΔH = J/mol
Note that no pressure is given with the normal boiling point. This is because, by definition, the vapor pressure of a substance at its normal boiling point is 760 mmHg.
The molar enthalpy of vaporization of hexane (C6H14) is 28.9 kJ/mol, and its normal boiling point is 68.73 °C. Calculate the vapor pressure of hexane at 25.0 °C?
T1 = 25.0 °C + 273 = 298.15K, T2 = 68.73 °C + 273.15 = 341.88K, P2 = 760 mmHg, ΔH = 28.9kJ/mol x 1000j/1kj = 28900J/mol
ln(760/P1) = 28900/8.314 (1/298.15 – 1/41.88)
P1 = 171 mmHg