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Un-ideal Gas Equation!

  1. Sep 20, 2009 #1

    I am trying to find a formula which can calculate the volume that gas X will decrease by when pressure Y is added to it. Now I have already been pointed to the Ideal Gas Equation, however I read that this is best suited when looking at a single volume of a monatomic gas eg. Argon.

    However I wish to experiment (play about) after doing my calculations and instead of messing about with a monatomic gas I would much rather use air so my question is...

    Does anyone know a formula I can use to calculate the change in volume of a mixed gas (air) with varying pressure.

    PS. I guess a list of pressure volume ratios for air which I could extrapolate from would also be usefull if anyone has them?...but I'd rather do the calculations myself.

  2. jcsd
  3. Sep 20, 2009 #2


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    Science Advisor

    Well, the next-simplest after the ideal gas law is the van der Waals gas law (which has a pair of parameters). There are some other theoretical models, but I'd suggest a Virial expansion. (I believe that's the mostly used in practice.) It depends really on what temperature/pressure range you're interested in as well.

    Note that most equations of state (including the vdw) can get pretty bad results at or near a phase change point.

    Hmm, annoyingly the CRC handbook I've got only lists vdW coefficients, and not for air. Shouldn't be hard to find though.

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