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Second derivative of Pressure with respect to moles?

  1. Dec 1, 2009 #1
    binary mixture.
    Na=moles of a
    Nb=moles of b

    (using Peng Robinson Equation of state)

    (second order partial derivative below)

    d^2P/(dNa^2) holding T, molar volume, Nb constant

    I can't figure out how to do this?

    I know that Peng Robinson is a function of concentration of Na and Nb... but I tried to substitute all molar fractions with Na/(Na+Nb) & Nb/(Na+Nb) put the complete expression in Mathcad (was huge!) and it gives 0 as the first derivative of Na or Nb so something went wrong or it decided not to do it...

    Attached is/was my Mathcad input and also the whole equation I am trying to get an expression of (this partial derivative is just part of it; don't mind the cursor on the right)

    Attached Files:

  2. jcsd
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