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Homework Help: Ideal gas law-

  1. Feb 17, 2009 #1
    1. The problem statement, all variables and given/known data

    A child holding a helium filled balloon @ sea level (T= 20 C) lets go of the string. The balloon rises freely several thousand meters, where T = 5 C and P = 0.70 atm. Find the percentage change in the balloon's volume.


    2. Relevant equations

    PV = nRT

    percentage change = amount of change (amt change = V2-V1)/ original amount (V1)

    3. The attempt at a solution

    Is my reasoning correct on this one? Since n and R don't change with altitude, temperature or volume, I solved for volume: V = P/T Then I solved for V1 and V2. Lastly used the above % change equation.

    The only thing that is bothering me about my method is that I'm not sure how the units work out for the volume equation I posted. I guess Im figuring it jut works out (???) But, I really need to understand it for exams :wink: I see no other way to work this problem. Also- is the % change formula correct?

    Thanks.
     
  2. jcsd
  3. Feb 17, 2009 #2
    Looks good to me.

    Units? Try working them out.
     
  4. Feb 17, 2009 #3
    V = P/T

    m^3 or liters = Pa or (N/m^2) / (C)

    I'm still don't understand how it works out. Do you mean work it out with the n and R included?
     
  5. Feb 17, 2009 #4
    When you are calculating percent change in volume, that is a ratio and there are no units; they cancel out.

    However, when working out the units in the ideal gas law, you will need to include nR, (or Nk):

    PV=NkT=nRT
     
  6. Feb 17, 2009 #5
    Thank you BF.
     
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