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Homework Help: Chemical thermodynamics, entropy, Maxwell relation and Z

  1. Oct 21, 2016 #1
    1. The problem statement, all variables and given/known data

    A gas obeys the equation of state

    z=1/(1-b(molar density))

    What is the molar entropy change of expanding this gas from a volume of 1 dm3 to 2 dm3 at a temperature of 300 K, given b = 0.2 dm3? Calculate your answer to three significant figures and select one answer from the list below.

    You may wish to use the Maxwell relation

    (∂p/∂t) = ∂s/∂v LHS constant V RHS constant T

    2. Relevant equations

    PV=RT maxwell realation possibly z = PV/RT

    how can I solve for the molar density to solve the compressibility factor. this is an example exam question so I already know the answer is 6.74 J

    3. The attempt at a solution

    I have substituted the LHS of the maxwell relationship for R/T (not sure if this is a correct assuption) then intergrated to get Rln(V2/V1)= change in entropy

    this gives me 5.76 I'm assuming that this then needs multiplying by Z to get the correct answer of 6.74.

    I have attempted to solve for z using the 1/volume to get molar density using both densities which give the incorrect answer. I have then tried to average the Z of both (Z1+Z2)/2 which again was a long shot, it was close but I'm assuming that is not the way its done because it seems very inaccurate to use an average for compressibility.

    Again this is not a coursework it is a multiple choice practice exam question, that gives the solution, just no feedback.
  2. jcsd
  3. Oct 22, 2016 #2
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