[nitram0606]

Hi,

This problem is giving me trouble since last night...

As Follows:
The density of air 20km above the earths surface is 92g/m^3 and the pressure is 42mm Hg, at a temperature of -63°C.

a) What is the average molar mass?
I figured this out pretty easily.  Converted so that I could use PV=nrt and came up with total of .003204607 moles and average Molar Mass of 28.7 grams/ mol.

The second part is where I'm lost....
b) If the atmosphere at this altitude consists of only O2 and N2 what is the mole fraction of each gas?

I must be missing something because I have total moles, but how could I know the mole fraction without the % or moles of O2 or N2 individually?


Thanks!
Martin 

[curiouscat]

If it were 21% O2 what would be the avg. MW?

Do that in reverse.

[nitram0606]

If it were 21% O2 what would be the avg. MW?

Thanks, I'm trying it out I'll post how it goes.

Sincerely,
Martin

[nitram0606]

Well,

I got that answer.... by saying (.21x32g/mol) + (.79x28g/mol) = 28.84 or 29g/mol for air.

I'm still not quite sure how that relates to my problem though, how do I get to that point from here?
Another hint would be awesome, I'd always prefer to figure it out with hard work rather than given the answer.

Sincerely,
Martin

[Borek]

What are the numbers 0.21 and 0.79 that you used? Can you use them again?

[nitram0606]

I'm not sure I know what you mean.

The 21% and 79% were just given to me by the other user to go from % to average molar mass, unrelated to my problem.

I guess my question is:
Given mols of a gas, and then being told it's composed of O2 and N2 and knowing it's 28.7g/mol average MM, how do I get to the mole fraction without knowing the % of each gas?

Still not getting this in my brain.

Sincerely,
Martin

[Borek]

You can't calculate molar fractions without using some additional information, just like you can't calculate average molar mass without using some additional information about the air composition. It happens that it is exactly the same information that is needed in both cases - so if you have used it for the average molar mass, why don't you use it for molar fractions?

[nitram0606]

You can't calculate molar fractions without using some additional information, just like you can't calculate average molar mass without using some additional information about the air composition. It happens that it is exactly the same information that is needed in both cases - so if you have used it for the average molar mass, why don't you use it for molar fractions?

Yeah I understand that, but what I don't know is how to create the relationship given a gas information (pressure, volume, moles, temperature) and density of the gas?  Can someone point me in a more obvious right direction please.

Thanks,
Martin

[nitram0606]

Hi,

The density of air 20km above the earths surface is 92g/m^3 and the pressure is 42mm Hg, at a temperature of -63°C.

a) What is the average molar mass?
I figured this out pretty easily.  Converted so that I could use PV=nrt and came up with total of .003204607 moles and average Molar Mass of 28.7 grams/ mol.

b) If the atmosphere at this altitude consists of only O2 and N2 what is the mole fraction of each gas?
Can't figure this part out from what I'm given....

Ok, well I'll repost the actual question and if someone wants to take a look at the actual question that i'm trying to answer and let me know how they would do PART B then that would be great, otherwise i'll keep trying to figure it out on my own.

Thanks,
Martin

[nitram0606]

Well it was insanely simple...

x + y  = 1  (molar fractions must = 1 obviously)
Then 32 x + 28 x = 28.68 grams / mol average mass... and then I solved for x and y.

Thanks anyways,
Martin