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Homework Help: Air Molecules at STP

  1. Apr 12, 2010 #1
    1. The problem statement, all variables and given/known data
    From the average separation between air molecules at STP, and their mean speed, estimate how long it would take one molecule to move into the region occupied by another. Assume that air consists mainly of nitrogen molecules.


    2. Relevant equations
    v(rms)=sq root(3kT/m)
    molecular mass of nitrogen is 28u


    3. The attempt at a solution
    Do i just ignore oxygen and just work on nitrogen molecules?
    I dont know how to find the average separation of air molecules at STP.
    the average speed of a nitrogen molecule works out to be:
    v=sq root([(3)(1.38x10^-23)(273)]/4.6x10^-26
    v=462m/s
    what should i do next?
     
  2. jcsd
  3. Apr 12, 2010 #2

    Borek

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    Staff: Mentor

    Imagine for a moment all molecules don't move and each sits in the center of its own cube. What is volume of this cube? What is a distance to each of the neighbors?

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    methods
     
  4. Apr 12, 2010 #3
    ok, so i read in my course notes that the average distance between nitrogen molecules at STP is 3.34x10^-9m because one mole of gas at STP contains 6.02x10^23 molecules and the total volume is 22.4L. So you divide the volume by the number of molecules to get the distance right? and then to get the time it takes to move, you just multiple the velocity by the distance?
    so 462m/s x 3.34x10^-9 = 1.54x10^-6 seconds
    does this make sense?
     
  5. Apr 12, 2010 #4

    Borek

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    Staff: Mentor

    Logic looks OK to me.

    --
    methods
     
  6. Apr 12, 2010 #5
    thank you for the help
     
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