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Homework Help: Thermodynamics Pressure Calculation

  1. Sep 19, 2012 #1
    A spherical balloon is filled with helium atoms (Note: 1 atm = 101.3 kPa).
    What is the total translational KE of the atoms if the balloon has a diameter 26.0 cm at 46.0 °C and the pressure inside the balloon is 121.6 kPa?

    The answer is 1680J but I can't get it.

    I used KE=3/2kT (in J/molecule) and then multiplied that number by the number of molecules PV=nRT. Am I working it out wrong or using the wrong numbers (like should I use Kelvin for temperatures)?

    KE=3/2*1.38x10^-23*46 =2.4x10^21
    V= 4/3*pi *radius^3 =4/3*pi*(13/100)^3 =9.2*10^-3
    n=PV/RT = (121.6*9.2x10^-3)/(8.314*(46+273)) Not sure if I should use Kelvin here
    n= 4.22x10^-4
    KE*n = 2.4x10^21* 4.22x10^-4=1x10^18J (which is so off the right answer).
  2. jcsd
  3. Sep 19, 2012 #2
    Oops. Somehow my thread got posted twice...!?! Sorry...
    I dunno how to delete...
  4. Sep 19, 2012 #3
    never mind. I got it. Thanks for looking anyway.
  5. Sep 19, 2012 #4


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