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Homework Help: Manipulating Pv=nRT equation

  1. Jan 22, 2015 #1
    1. The problem statement, all variables and given/known data
    A balloon initially contains 20 grams of helium at a pressure of 1000 torr. After some helium is let out of the balloon the new pressure is 900 torr, and the volume is half of what it was. If the temperature has not changed, how much helium is now in the balloon?

    2. Relevant equations

    3. The attempt at a solution
    I first converted the grams to moles and the pressure to atm. I attempted this problem by setting the first and after values to each other. I used PV/n = PV/n (before and after). When I set these equal to each other I get approx. 4.6 mol He or approx. 9 grams He. Is this a legal way to approach this problem?
  2. jcsd
  3. Jan 22, 2015 #2


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    Science Advisor
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    Gold Member

    Conditionally. The notation is not what it could be. Subscripts, insertion of masses in the appropriate places would improve things.
    It's not obvious that you've done anything without some remarks about initial and final "states" and values.
    Don't overuse the word "approximate" or its derivatives or abbreviations when you've got an exact answer based on the problem statement.
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