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Homework Help: Conceptual Van der Waals Question

  1. Nov 17, 2013 #1


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    Gold Member

    1. The problem statement, all variables and given/known data

    Why does the Van der Waals equation adjust pressure up and volume down?

    2. Relevant equations

    (P + a)(V-b) = nRT (overly simplified, but it gets the point across).

    3. The attempt at a solution

    Okay. Volume refers to total free volume in a container in the context of the gas law. I can understand why volume is adjusted down relative to the ideal gas law equation (PV = nRT). Gases aren't point masses and actually occupy some volume.

    However, why is pressure adjusted up? Gas molecules may attract and participate in inelastic collisions. This would seem to decrease the actual pressure. Why then would pressure need to be adjusted up?
  2. jcsd
  3. Nov 19, 2013 #2


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    Staff: Mentor

    If you were to measure the pressure of a gas with ##a = 0##, and then, with everything else the same, with ##a > 0##, how would the measurements differ?
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