How to Calculate Vapor Pressures at Other Temperatures
- 1). Look up the standard enthalpy of vaporization and standard entropy of vaporization for the liquid in which you are interested.
- 2). Determine the conditions at which you want to know the vapor pressure of the liquid. You will need to know the temperature, the information from Step 1 and value of the gas constant R.
- 3). Convert all conditions to SI units. The temperature units are Kelvin, the standard enthalpy of vaporization is in Joules/mole, the standard entropy of vaporization is in Joules/(Kelvin * moles) and the gas constant is 8.314 J/K*mol.
- 4). Plug values into the Clausius-Clapeyron equation, which is: ln (Pvap2/Pvap1) = (Hvap/R)(1/T1 -- 1/T2), where the partial pressure of vaporization of water at temperature 1 is Pvap1 and partial pressure of vaporization of water at temperature 2 is Pvap2. Hvap is the standard enthalpy of vaporization, 40.7 kJ/mol. Evaluate the terms on the right side of the equation. Raise both sides of the equation to a power of e and the equation becomes Pvap2/Pvap1 = e^(H^o / R)(1/T1 -- 1/T2). Multiply both sides by Pvap1 and the equation simplifies to Pvap2 = Pvap1 * e^(H^o / R)(1/T1 -- 1/T2).