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Distance between molecules

  1. Mar 6, 2009 #1
    1. The problem statement, all variables and given/known data

    given that in a system, there are 5 % oxygen and 8 % nitrogen. it's also given the molecular mass of the two gasses. find the distance between molecules

    2. Relevant equations

    3. The attempt at a solution

    i don't know what the percentage is for. is it the percentage of mass, volume, or mole?

    i even don't see the connection between the data given and the question asked T_T
  2. jcsd
  3. Mar 6, 2009 #2


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    Maybe you should figure out how many molecules there are in a volume?

    For any particular volume, of a given (molar) mass and %'s of objects (molecules in this case) with particular mass then how close together are they on average to fit in the volume?
  4. Mar 6, 2009 #3
    how can we identify that the percentage given is molecules?

    i suppose that the distance between the molecules is the distance between oxygen molecules and nitrogen molecules? or can it be the distance between oxygen molecules itself?

    i guess maybe to get the answer is from volume. using V = 4/3 phi r^3, we find radius of oxygen and nitrogen molecules. then to find the distance, just simply add those radius

    but i can't find the volume. From your idea, i should find the number of molecules and i even can't get there...

    if the given data is percentage of molecules, i think we need to know how many molecules there are in the given system, but there is no such information on the question..

  5. Mar 6, 2009 #4


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    Perhaps if you post the entire statement of the problem with the specifics?
  6. Mar 6, 2009 #5
    i don't know the entire statement of the questions because i got it from friend and she also doesn't remember but all the specifics have been stated.

    the information given are the percentage and the molar mass
  7. Mar 6, 2009 #6


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    OK then where is the other 87%?

    What is the other 87%

    What is the volume?

    How many moles? Or failing that how many moles per liter?

    Without the complete question - and I continue to think there is more than what you are getting second hand - I see little direction toward a solution.
  8. Mar 7, 2009 #7
    hm... maybe the question isn't complete

    so i conclude that based on the previous data, we can't solve the problem.

    but is my idea right?

    we find the radius and add them?

  9. Mar 7, 2009 #8


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    Not exactly.

    I think you want to look at it from the point of view of the number of objects in the overall volume. These are apparently gas molecules flying about and colliding. I think the radii of the molecules will turn out to be a lot smaller than the average distances. Hence the average distance will be more likely determined by the average volume of an object ... i.e volume / # items.
  10. Mar 8, 2009 #9
    if the question turned out like this :

    given that in a system there are 80 % of oxygen and 20 % of nitrogen. assume that the system in STP condition.

    can we find the distance between molecules or we should know the volume of the system ?

    so the distance between molecules = volume / number of molecules ?
    but the unit will be m^3

    will it be different if we take the shape of the volume to be cube and if we take sphere ?
    Last edited: Mar 8, 2009
  11. Mar 9, 2009 #10


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    Since you are not dealing with beach balls packed in a crate, I'd say so long as atomic volumes are << than the average m3 domains then I think a cubic box will more accurately determine their range. Since they are averagely placed in the boxes, then I'd describe their distances as merely (on average) the length of a side of a cube they are in.
  12. Jun 29, 2009 #11
    ok, thx a lot lowlypion ^^
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