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chem question

Discussion in 'MCAT Study Question Q&A' started by masterMood, Jun 20, 2008.

  1. masterMood

    masterMood Banned Banned

    Jun 25, 2004
    76. from examkrackers lecture 4.

    The air we breahte is approximately 21% O2 and 79% N2. If the partial pressure of nitrogen in air is 600 torr, then all of the following are true:

    B. The mass of nitrogen in a 22.4 sample of air is 22.1 grams at 0 degrees Celsius.

    The book says its true, and the explanatin is that

    "Standard Molar Volume is the volume occupied by one mole of any gas at STP"

    so if that's true then wouldn't there be one mole of N2 (g) or 28 grams instead of 22.1 grams?
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  3. nfg05

    nfg05 2+ Year Member

    May 19, 2008
    What you're forgetting about is that air is only 79% N2, as stated in the question. So in one mol of air (which equals 22.4 L of air at STP), there are actually only 0.79 mol of N2. (0.79 mol)(28 g/mol) = 22.1 g/mol.
  4. Quantum Chem

    Quantum Chem 2+ Year Member

    Jun 10, 2008
    College Park, Md
    Air is 21% O2 by mole and 79% N2 by mole. If you have 22.4L of an air sample, you would have one mole of air. You would then multiply .79 moles by 28 g/mole to get 22.1g of N2 and you could do the same to get the mass of O2.
  5. masterMood

    masterMood Banned Banned

    Jun 25, 2004
    thanks guys!
  6. Yolisept


    Jul 23, 2013
    The explanation also says that the percentages given are by particle and NOT by mass, so we can't actually use that logic to eliminate B. B is dependent on the percentages being given by mass not by particle.

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