Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Constant C in Vapor Pressure Equation

  1. Sep 29, 2009 #1
    I'm working on vapor pressure for my general chemistry class, and I recently came across the curve P=C^(-[tex]\Delta[/tex]H/RT). I was wondering what the units are for C (or are there no units because it is a constant?) and where I could find a table of values for common liquids (water, ethanol, methanol, diethyl ether, etc.). I know that I can cancel out C when actually doing problems, but I was curious to see the magnitudes of the values and how much they differ from each other. I haven't been able to find a table of values so far, so if anyone has a link I'd appreciate it.
  2. jcsd
  3. Sep 29, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The complete expression is

    [tex]P=P_0\exp\left(\frac{\Delta S}{R}-\frac{\Delta H}{RT}\right)=P_0\exp\left(\frac{\Delta S}{R}\right)\exp\left(\frac{-\Delta H}{RT}\right)[/tex]

    where [itex]P_0[/itex] is the unit pressure (e.g., 1 Pa, 1 bar, 1 atm, etc.) and [itex]\Delta S[/itex] is the entropy increase when the condensed phase evaporates. This equation is derived by setting equal the chemical potentials of the condensed and vapor phases.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook