Calculate the percentage of nitrogen in air

[zorro]

Homework Statement

At 0C and 1 atm pressure the densities of air, oxygen and nitrogen are 1.293, 1.429 and 1.251 kg/m3. Calculate the percentage of nitrogen in air, assuming only these two gases to be present.

The Attempt at a Solution

Which equation should we use here?

 

[zorro]

No one knows the solution??

 

[Borek]

Gas volumes are additive. If you mix some volumes of oxygen and nitrogen, final volume will equal sum of volumes, also final mass will equal sum of masses. Using this information try to express the density of the air as a function of composition (say assuming x fraction of nitrogen, and 1-x fraction of oxygen). Solve for x.

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[zorro]

I tried to derive an equation but got stuck.
Can you just write the equation? That will help me in understanding the problem.

 

[Borek]

I would have to derive it by myself, I didn't.

Please show how you started and where you got stuck.

 

[zorro]

V = V_O2 + V_N2
n_T/P = n_O2/P_O2 + n_N2/P_N2 ( as temperature is constant )
P = 1atm
Dividing the equation by n_O2 + n_N2
We get mole fractions on both sides.
On solving I get P_N2 + P_O2 = 1
which I don't think is of any use.

 

[Borek]

V = V_O2 + V_N2

OK

n_T/P = n_O2/P_O2 + n_N2/P_N2

I told you to write mass balance - that is, express mass of each gas as a function of its volume and density. Pressure and temperature are of no use here.

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[zorro]

Thanks a lot Sir!
I got it

 

[Borek]

Don't Sir me.

You may call me Mr. pH if you need to use a title.

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[zorro]

Sure, Mr. PH