# Why Nitrogen percentage is approx.71% at ground level?

Discussion in 'General Science & Technology' started by plakhapate, May 29, 2007.

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1. ### plakhapateBannedBanned

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The molecular wt. of the gases are as follow

N2 = 28, O2 = 32, CO= 28, CO2=44, H2O= 18

Low mol. wt. means lighter gas.

Hence CO2 shall remain at ground level.

Oxygen shall remain above CO2.

N2 and CO shallbe above O2,

and water shall go at top.

However due to variation in temp. turbulance is created.

But why the percent of N2 and O2 are stabilised to 79 and 21 approx.at ground level ?

It can be anything depending on the turbulance.

Pls comment.

P.J.LAKHAPATE
plakhapate@rediffmail.com

Last edited: Jun 1, 2007
2. ### dexterROOTRegistered Senior Member

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Um, I dont know for sure. . . but I'm pretty sure it's because its all in gasious form. And they are within an atmosphere. and since they are all in the same space, they share the same are in which they can 'bounce'.

3. ### draqonBannedBanned

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...bacteria that make nitrogen from ammonia...anyone?

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No, it cannot be just "anything." The atmosphere is a HIGHLY dynamic place - there is constant mixing and remixing. We, as surface dwellers aren't fully aware of it but just a few feet above our heads the air is in constant motion. Even in the doldrums where the air appears to very, very still it actually isn't - it's rising straight up (and mixing).

5. ### D HSome other guyValued Senior Member

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Even turbulance is not required. The propensity to mix due to Brownian dynamics vastly overwhelms the propensity to separate due to gravitational dynamics.

Suppose you create an artificial atmosphere in a closed vessel, with the constituent gases carefully layered as outlined in the initial post. It will not take much time for the gases to become thoroughly mixed due to Brownian motion if the gases are anywhere near room temperature.

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Very true. Sometimes called the "Drunkard's Walk" because of the total randomness of the motion.

7. ### plakhapateBannedBanned

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Even if we assume complete mixing is there , why the composition remain as 79:21 and not any other ratio?

P.J.LAKHAPATE
plakhapate@rediffmail.com

8. ### SciencelovahRegistered Senior Member

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4,349

imo, it has nothing to do with the molecular weight. Its with the number of
the molecules available. There are more N2 molecules than O2 molecules in
the atmosphere (780,840 ppmv vs 209,460 ppmv).

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It's because our planet is a living thing and that's the current balance it has struck. That ratio is FAR from cast in stone and unvarying. It was considerably different in the distant past and will most likely be different in the distant future.

10. ### James RJust this guy, you know?Staff Member

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In any gas at room temperature, the molecules are zipping around like crazy, just due to their temperature. Gravity's effect on the nitrogen and oxygen molecules in your room is negligible. And the same applies to the atmosphere, more or less. If you calculate the average speed of an oxygen molecule at room temperature, it is on the order of hundreds of metres per second. If it is in a room, it is bouncing off the walls, the ceiling, the floor, other objects, and (most often) other gas molecules many times per second.

The ratio of oxygen to nitrogen in the atmosphere has nothing to do with gravity. It is just a straight count of the relative numbers of molecules that exist.

As Read-Only has said, the numbers of molecules that exist is determined partly by how many there were went the Earth formed. It also depends on volcanic activity, and many other processes that can take molecules out of the air or put them into it. The effects of life are major on our planet. The oxygen content would be nothing like what it is without the presence of life. In fact, if you're looking for life on other planets, oxygen content in the atmosphere is a major clue.

11. ### phlogisticianBannedBanned

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er,

pv= 1/3 nm<c^2>

or something? 'c' being the 'mean free path' iirc?

It's been a while, but some of it stuck.

12. ### D HSome other guyValued Senior Member

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For an ideal gas with molar mass M,

$\bar v = \sqrt{\frac{8RT}{\pi M}}$

13. ### KlippymitchThinkerRegistered Senior Member

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Because when heat is added to Nitrogen it's pressure doesnt change or at least drastically. Like for example Co2

Co2 at 80*f is about 950psi.... Co2 at 40*F is about 550psi

Nitrogen at 80*F in pressurized in a container will be lets say we pressurized the container to match the pressure of Co2. So at 80*F nitrogen is 950psi. And at 40*F Nitrogen is still 950psi. It doesnt really change much with temperature.

So co2 is flying all around becuase of the heat from the sun and the nitrogen just stays the same. Hence why it is what it is.