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Homework Help: Physical Chemistry-Gas Laws (Find R and M)

  1. Sep 25, 2010 #1
    Thanks ahead of time for any help!

    1. The problem statement, all variables and given/known data
    From Atkins Physical Chemistry text, 9th Edition:
    1.7b) The following data has been obtained for oxygen gas at 273.15 K. Calculate the best value of the gas constant R from them and the best value of the molar mass of O2 (oxygen).

    p(in atm)
    P1=0.750 000
    P2= 0.500 000
    P3=0.250 000

    Vm [molar volume] (in dm^3/mol)
    Vm1= 29.8649

    2. Relevant equations
    3. The attempt at a solution

    To find R, I used R= (Vm P)/ T to find the different Rs for each set of data, then used y=mx+b (using data points [P, R]) to extrapolate back to when p=0 (when ideal gas is most accurate; so that means the y intercept which was equivalent to R).

    I know you can just graph it and get the same result. Either way, I got R= 0.0820614 dm^3 atm/K mol.

    Now I'm lost. How do I set it up to find molar mass? Do I extrapolate using a graph again? I'm not given density, so it throws a wrench in a lot of the equations I tried using, like:
    Vm= M/ density = RT/P= V/n
  2. jcsd
  3. Sep 25, 2010 #2


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    Staff: Mentor

    No idea - I can't think of any approach that will let calculate molar mass from the given data.

    That's to let you know someone actually read your post and spend a moment thinking how to help.

    It can always be an error in the book. I can be also missing something.
  4. Sep 25, 2010 #3
    Thanks. I know that in the old editions, density is given, but density was omitted for the 9th edition. Not sure if it was a mistake or if it was to make the question trickier!
  5. Sep 25, 2010 #4


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    Staff: Mentor

    Must be mistake then.

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