1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Science Advisor
    Homework Helper

  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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar

    Staff: Mentor

    Pounds? Talents? Stones? Carats? Maunds? Catties? Wiarduneks?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook