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Homework Help: Isothermal expansion

  1. Dec 12, 2009 #1
    1. The problem statement, all variables and given/known data
    Suppose 161 moles of a monatomic ideal gas undergoes an isothermal expansion as shown in the figure (attached). The horizontal axis is marked in increments on 20 m3

    What is the temperature at the beginning and at the end of this process?

    2. Relevant equations
    PV = nRT

    3. The attempt at a solution
    Pi = 400 kPa
    Vi = 20 m3
    Pf = 100 kPa
    Vf = 80 m3

    T = (PV)/(nR) = (400*20)/(161*8.31) = 5.98 K

    My answer is wrong according to the online homework grading system. What am I doing wrong?
    Any help is appreciated.

    Attached Files:

  2. jcsd
  3. Dec 12, 2009 #2


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    Homework Helper

    The pressure is kilo pascal.
  4. Dec 12, 2009 #3
    OK. But even if I convert kPa to Pa and calculate, that's still not the answer.
    T = (400,000 Pa * 20) / (161*8.31) = 5979 K
    This seems outrageously large.
  5. Dec 13, 2009 #4

    Andrew Mason

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    Science Advisor
    Homework Helper

    True. But you can blame the person who drafted the question. That is the right answer to this question.

    One mole of a gas at STP occupies 22.4 litres or .0224 m^3. That is at about 100 kPa pressure and a temperature of 273 K. Here, you have 161 times that amount of gas occupying 80/.0224 = 3571 times as much volume. So to reach the same 1 atm pressure the temperature has to be 3571/161 = 22 times the temperature at STP -about 6000 degrees.

  6. Dec 13, 2009 #5
    Okay. Thanks for your help.
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