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Homework Help: Vapor pressure

  1. Apr 13, 2008 #1
    Hello everyone!

    I've encountered a small problem with one of our "special" assignments.

    The normal boiling point for ethyl alcohol is 78.4 C, S for C2H5OH(g) is 282,7 J/mol*K.
    At what temperature is the vapor pressure of ethyl alcohol 357 mm Hg?

    Maybe I'm stupid, but I can't find a connection between the pressure and entropy. Or the phase transition. Original I thought it had something to do with Gibbs vs equilibrium, but now I'm even more confused.

    I'd love to do the problem myself but I'm in need of a hint/tip :)

    Thanks in advance.
  2. jcsd
  3. Apr 13, 2008 #2


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    The only equation that comes to mind at the moment is some type of derivative of the Clayperon equation.
  4. Apr 14, 2008 #3


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    Yes, this is definitely a Clapeyron relation problem. The change in characteristic pressure of a phase transformation with temperature is related to the enthalpy of the transformation. And the enthalpy change is related to the entropy change. Let us know if you get stuck.
  5. Apr 16, 2008 #4
    Thanks for the help.

    I'll post my calculations later today, still not entirley sure I got it right.
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