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Homework Help: Calculating final pressure

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data

    As a 1.00 mol sample of a monoatomic gas expands adiabatically the work done on it is
    -2.50x10^3J. The initial temperature and pressure of the gas are 500k and 3.60atm. Calculate
    a. The final temperature and b. The final pressure.

    2. Relevant equations

    First law of thermodynamics
    Eint = Q + W

    3. The attempt at a solution
    I was able to solve the first part of the question. Since it was an adiabatic process Q = 0 and then used Eint = nCv(Tb - Ta) to get the final temperature which is 300k.

    But i can't seem to get the final pressure. I have no idea on how to begin. Please any help will be very much appreciated.
  2. jcsd
  3. Oct 26, 2011 #2
    Use the state equation for an adiabatic process i.e. p*T^(k/(1-k))=constant. Where k is the quotient of the heat capacities.
  4. Oct 26, 2011 #3
    quotient of heat capacities?? please i don't understand that part!!!!
  5. Oct 26, 2011 #4
    Its just that k=C_p/C_v. The quotient of the heat capacities at constant pressure and constant volume.
  6. Oct 26, 2011 #5
    okay..i get that part.. is the constant R??
  7. Oct 26, 2011 #6
    No... that equation means that this quantity p*T^(k/(1-k)) is constant throughout the process. Which means that its the same in the beginning and at the end. U know the pressure and temperature at the beginning you know tht temp at the end...
  8. Oct 26, 2011 #7
    ok.. but i'm getting a wrong answer which is 5.9atm but it should be 1atm...
  9. Oct 26, 2011 #8
    Are you making your calculations correctly? whats the value you got for k for the gas?
  10. Oct 26, 2011 #9
    i got 1.5
    i used C_v to be 3/2R since the gas is monoatomic and C_p to be 5/2R
  11. Oct 26, 2011 #10
    shouldnt it be 1.667? And then whats the next step? show the whole calculation otherwise i cant see wtz wrong...
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