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Homework Help: Molecules in the Atmosphere

  1. Apr 15, 2004 #1
    I was just wondering if you know the height of the atmosphere, and the pressure that atmosphere creates on the Earth (1 atmosphere), then how would you approach calculating the number of molecules in the atmosphere?
    I was thinking of using the PV=NKT formula, where K is boltzmann's constant.
    I'm not sure if I can do this, but this is what I did.
    The only information I needed to calculate N was the Volume. So because I knew the height I simply calculated the volume of the Earth plus atmosphere and the volume of the Earth. Then I subtracted the volume of the Earth from the volume of the Earth plus the atmosphere.
    I get a value of an order of magnitude 10^44 molecule.
    By the way the Temperature is 0 degrees celsius.
    Can anyone tell me if what I did was ok or if what I did was wrong?
  2. jcsd
  3. Apr 16, 2004 #2
    What kind of molecules? The water molecule is really tiny and more of them could crowd in there. I guess we could always count them, ha ha...one....two...three...
  4. Apr 16, 2004 #3


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    Molecular density of ideal gasses is independent of specie. It is always 22.4 l/mole at STP. You could use the ideal gas laws, and integrate to higher altitudes which will have lower pressures (first calculate P as a function of altitude). This will work until you get to the stratosphere, where the chemical composition of the atmosphere changes.

  5. Apr 16, 2004 #4


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    Why would it stop working there? I thought it was independent of the specie.
  6. Apr 17, 2004 #5
    What if you are assuming that density is constant up to some altitude?
    Would my way be all right then or is it still wrong?
  7. Apr 17, 2004 #6
    The universal gas law is independent of the specie, but the volume of gas has to be a homogeneous distribution of whatever the specie (or spcecies) is.
  8. Apr 19, 2004 #7
    So can anyone tell me if what I did was ok if you assume that the density is constant up to some altitude h?
  9. Apr 20, 2004 #8


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    Something just doesn't seem to be clicking in my brain. I suppose my first logical question should have been:

    "How can a change of chemical composition be maintained across the boundary between the stratosphere and the (troposhere?)?"

    Shouldn't the species of these two regions diffuse into each other (for that matter, shouldn't they have done it a long time ago) and so homogenize? What prevents/counteracts this?
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