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Homework Help: Volume using Compressibility Factor

  1. Feb 5, 2013 #1
    1. The problem statement, all variables and given/known data

    Find the volume of 2 kg of ethylene at 270 K, 2500 kPa using Z

    2. Relevant equations

    Method to Solve for Z, Using Tr and Pr

    PV = ZnRT

    3. The attempt at a solution

    Tr = 0.9561
    Pr = 0.496
    Z found to be approx. 0.75
    R given on a table at 0.2964 kJ/kg K

    From here, I should have all information needed, I was just confused on units at this point.
    V = ZnRT/P
    V = (0.75)*(n)*(0.2964 kJ/kg K)*(270 K) / (2500 kPa)

    At this point, my main questions are (probably very simple):

    What is n, and why? My best guess: (2000 g)/(28 g/mol) = 71.43 mol

    What are units for P? My best guess: atm, -> 2500 kPa*24.67 atm/kPa = 61675 atm

    Using these numbers, I get Volume to be way too low, corrections needed somewhere. Help appreciated.
  2. jcsd
  3. Feb 6, 2013 #2
    Take a look at your units. If you're using kj/kg K for R, you should use equation:
    PV = mRT, where R is the gas constant for that gas and m is the mass in kilograms. If you use PV = nRuT (Where Ru is the universal gas constant), then you'll need to use the number of moles.

    Also, use the fact that 1 Pa*m^3 = 1 J.
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