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Molar Volumes

  1. Nov 17, 2015 #1
    Guys I have a doubt or rather a misconception
    It is as follows:
    Consider a container with a partition having a gas A occupying 2/3rd of the volume and gas B occupying 1/3 rd of the volume
    Now suppose we remove the partition .As a result,gas A will occupy the whole volume of the container and gas B will also occupy the whole volume of the container(since the gas molecules are far apart and it doesn't matter if molecules of different gases collide as the size of the molecules is considered negligible)
    Now gas A and gas B should not contain the same number of molecules(since gas A occupied more space and gas B less)
    But doesn't this contradict avogadro's law
    That equal volumes of gases will contain an equal number of molecules??
    And how will you explain the composition of air by this way
    Like if the amount of air present in a room is say 5 litres
    The amount of nitrogen and oxygen should also be 5 litres (the volume of the room)
    So how does the composition of air (nitrogen 78% by volume and oxygen 21% by volume make sense??)
    I might have overlooked some things
    But I would be extremely grateful if those things are brought to my notice
    Help is much appreciated!!:)
  2. jcsd
  3. Nov 18, 2015 #2


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    Staff: Mentor

    What is meant by "the volume occupied by a gas" is the partial volume. If you have 5 l of air, the volume of nitrogen is 3.9 l and the volume of oxygen is 1.1 l.
  4. Nov 18, 2015 #3


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    Science Advisor

    Avogadros law states that gasses occupy the same volume under the same pressure. In the case of mixtures you have to use partial pressure, instead.
    In your example partial pressure of A is ##p_A=2/3 p## and of B is ##p_B=1/3 p##. Then ##n_A=p_AV/RT## and ##n_b=p_BV/RT##.
  5. Nov 18, 2015 #4
    But if there is a container with a capacity of 5 litres which has air In it
    Won't nitrogen and oxygen being gases occupy the entire space of 5 litres(although their amounts/molar concentrations)might be different
  6. Nov 18, 2015 #5
    That's fine
    But my doubt is if both nitrogen and oxygen are able to occupy 5 litres so how is their volume composition different??
  7. Nov 18, 2015 #6
    I've never understood why the terminology 79% by volume and 21% by volume is used, but, whenever I see it, I automatically substitute in my mind the words "mole percentage."
  8. Nov 18, 2015 #7


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    Staff: Mentor

    Imagine a cylinder with a thin, movable piston, separating both gases. One gas occupies some volume, the other occupies some other volume (as the piston can move pressures are identical on both sides). Now, remove the piston. Gases mix, and their partial pressures become lower than they were before, each one occupies whole volume now - but as these are in many aspects equivalent systems, for many applications we can still report the situation as if both gases occupied their initial volumes.
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