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Homework Help: Chemistry: Converting density to molar mass given other values.

  1. Sep 9, 2007 #1
    1. The problem statement, all variables and given/known data

    The density of a gaseous compound was found to be 1.23 kg/m^3 at 330K and 20,000 Pa. What is the molar mass of this compound.

    2. Relevant equations


    3. The attempt at a solution

    I don't think this is right but I assumed a volume of 1 m^3. So I used ideal gas law as follows:

    n = (20,000Pa * 1m^3)/(8.314 (Pa*m^3/mol.K) * 330K) = 7.29 mol.

    Then what I did was took density and multiplied it by the assumed volume of 1m^3 and got 1.23 kg/m^2. Then I took 1230g/m^3 and divided it by 7.290 mol. go get grams/mol*m^2. The answer was 168.72 but I can almost guarantee I am being retarded and overlooking something.
  2. jcsd
  3. Sep 9, 2007 #2
    you can also use this equation

    d = MP/RT

    let me know what you get
  4. Sep 9, 2007 #3
    I take it that is the molar volume formula?

    The reason why I didn't use that is because it says in my book that the formula for molar volume (above) is used under STP conditions. Did I read that wrong?
  5. Sep 9, 2007 #4


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    Rocophysics is right. [itex]\rho =\frac{\mu p}{RT} [/itex] for an ideal gas.
  6. Sep 9, 2007 #5
    Okay so d = MP/RT where M is the molar mass (or molar volume?)?

    Sorry I just want to make sure. And thanks for the help guys. Like I said I saw that formula in the book but I thought it said it only applied at STP.
  7. Sep 9, 2007 #6
    I ended up getting ~ 153g/mol
  8. Sep 9, 2007 #7
    that's not what i ended up getting, is this that the correct answer in the book? i'll re-do my calc.

    i'm off by ~ 15
    Last edited: Sep 9, 2007
  9. Sep 9, 2007 #8


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    I didn't get this answer either. I got the 168.72 g/mol that was given as the answer in the first post.
  10. Sep 9, 2007 #9
    lol i didn't even notice the answer in the first post, it's correct tho. different method but valid and good.

  11. Sep 9, 2007 #10
    Wait so I did it right the first time? I don't have the answer in the book, unfortunately it is an even numbered problem.

    Why didn't the formula you told me work?
  12. Sep 9, 2007 #11


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    It should have. Double check your calculation.
  13. Sep 9, 2007 #12
    M = d*R*T/P = 1230g/m^3 * 8.314 * 300k / 20,000 = 153.39 g/mol?
  14. Sep 9, 2007 #13


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    It's 330 K, not 300 K. :wink:
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