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Homework Help: Mass of oxygen

  1. Jan 21, 2010 #1
    1. The problem statement, all variables and given/known data

    What is the total mass (in grams) of oxygen in a room measuring 4.0 by 7.0 by 2.5? Assume that the gas is at STP and that air contains 20.95% oxygen by volume.


    3. The attempt at a solution
    I don't know where to begin.
     
  2. jcsd
  3. Jan 21, 2010 #2

    mgb_phys

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  4. Jan 21, 2010 #3
    Yes.
     
  5. Jan 21, 2010 #4
    Then you have everything you need except the variable n, or the number of moles. There's also an equation that relates n (number of moles) to the mass. Do you know of this equation?
     
  6. Jan 21, 2010 #5

    mgb_phys

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    Then that gives you the number of moles of air -
    Find the average molecular mass of air by assuming that the rest is N2 (mass=28)

    Then you know the number of moles of O2 - and so the mass
     
  7. Jan 21, 2010 #6
    V = (4*7*2.5) = 70m^3
    P = 1atm
    T = 273 K
    R = .000082
    n = PV/RT
    n = 3126.95
    1 mole of oxygen is 32 g/mol so
    32 * 3126.95 = 1000062.53 but you only want 20.95% of that so
    1000062.53 * .2095 = 20963.10
    = 2.10 x 10^ 4
     
  8. Jan 22, 2010 #7

    Borek

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    Pounds? Talents? Stones? Carats? Maunds? Catties? Wiarduneks?
     
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