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

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

 

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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'.

 

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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).

 

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.

 

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

 

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

 

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).

 

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.

 

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.

 

For an ideal gas with molar mass M,

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

Google Maxwell-Boltzman distribution for more info.

 

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.