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Homework Help: Isothermal and adiabatic please help

  1. Sep 4, 2010 #1
    isothermal and adiabatic please help!!!!

    1. The problem statement, all variables and given/known data

    I have been stuck on this problem for about an hour so any hep would be greatly appreciated!

    One mole of argon at 25 degrees C and 1 atm pressure is allowed to expand reversibly t a volume of 50 L (a) isothermally (b) adiabatically. Calculate the final pressure in each case assuming ideal gas behavior.
    The answers have been given and they are: (a) 0.489 atm and (b) 0.303 atm

    I can't get either a or b for some reason I don't know what I am doing wrong! Please help me :(

    2. Relevant equations
    Cv = 3/2 R Cp= 5/2 R
    Boyle's law

    3. The attempt at a solution

    my answers dont even come close...?
  2. jcsd
  3. Sep 4, 2010 #2

    D H

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    Re: isothermal and adiabatic please help!!!!

    Show some work please. It's impossible to show you where you went wrong if you don't show us what you have done.
  4. Sep 4, 2010 #3
    Re: isothermal and adiabatic please help!!!!

    nevermind i got it all -- I just didnt have my P and V in the right units.... anyway i found V1 using the ideal gas law, and the used boyle's law to find P2. Seems so simple when you have the correct units :)

    for part b i did PV(5/3)=nRT to find V1 and then used boyle's law to find P2. I got 0.296 for the answer tough. maybe my sig. figs. are off? i can't find out why.
  5. Sep 4, 2010 #4

    Andrew Mason

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    Re: isothermal and adiabatic please help!!!!

    You will have to use the adiabatic condition:

    [tex]PV^\gamma = \text{constant} \ne nRT[/tex]

    for part b). [itex]PV^1 = nRT[/itex]

  6. Sep 5, 2010 #5
    Re: isothermal and adiabatic please help!!!!

    Yeah, as AM has said, use the adiabatic condition, along with energy conservation to figure it out.
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