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The task: Determine the average molar mass of the atmosphere - each for the moist air and the dry air. One example that I have calculated is this one: I have the gas Nitrogen and the percentage for the moist air(77,0%) and the percentage for the dry air(78,08%) aswell as the molar mass(28,014) in $$\left(\frac{g}{mol}\right)$$

My solution: $$\text{Nitrogen}= 0,78 \cdot 0,28+ 0,77 \cdot 0,28 = 43,4 \frac{g}{mol}$$

Is that correct? Kind regards, iloveoov.

P. Siehr
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SAINT
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    This is certainly _not_ linear algebra. It looks like chemistry rather than math. – David K Oct 19 '17 at 14:33
  • I actually didnt know in what section I should put this question. – SAINT Oct 19 '17 at 14:34
  • It seems like a question for the chemistry stackexchange, keeping in mind their policies with regard to homework: https://chemistry.meta.stackexchange.com/questions/141/how-do-i-ask-homework-questions-on-chemistry-stack-exchange/142#142 – David K Oct 19 '17 at 14:42
  • Thanks for the advise. But I asked the question here, because I actually has this task in my math class. – SAINT Oct 19 '17 at 14:47
  • I think, that it is ok to ask it on a math board. In the same fashion as [this](https://math.stackexchange.com/questions/379927/how-long-will-it-take-marie-to-saw-another-board-into-3-pieces) is not on woodworking.stackexchange, [this](https://math.stackexchange.com/questions/637728/splitting-a-sandwich-and-not-feeling-deceived) is not on cooking.stackexchange, and [this](https://math.stackexchange.com/questions/11669/mathematical-difference-between-white-and-black-notes-in-a-piano) is not on music.stack-exchange. But I don't know the best choices of tags myself. A homework tag would be nice. – P. Siehr Oct 19 '17 at 14:48
  • If this came up in a math class, I would expect that it was immediately preceded by a bunch of definitions saying what "mol" means, what an "average molar mass" is, etc. (unlike in a chemistry class, where you might be expected to have learned those things weeks earlier). In the future I would recommend to make those definitions part of the question posted here. – David K Oct 19 '17 at 15:08
  • @PhilippSiehr Your first two examples don't assume any specialized knowledge of woodworking or cooking (in fact basically assume ignorance of those fields) and the third is prefaced by the statement that various experts in music _were unable_ to answer the question, which I'm sure could not be said about chemists and this question. But I will cop to having answered physics questions on MSE, partly because physics.SE has much more restrictive policies than chemistry.SE. – David K Oct 19 '17 at 15:11

1 Answers1

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The basic idea is correct. For moist air you can calculate the molar mass given by the part of nitrogen with $$M_{N_2,moist}=0.77\cdot 28.014\frac{g}{\text{mol}} = 21.57\frac{g}{\text{mol}}.$$

Note, that I multiplied with $28.014$ and not with 0.28. The next step would be to also do the same for all other gases (oxygen, argon, carbon dioxide, etc) and then sum that up: $$M_{air,moist}=M_{N_2,moist}+M_{O_2,moist}+ …$$ That will give you the molar mass of moist air. Afterwards do the same steps for dry air.

What you also did, is to sum both values of dry and moist air. That is not asked in your task.

P. Siehr
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  • Thank you for the answer now I finally understand what was actually meant. I appreciate that!! – SAINT Oct 19 '17 at 14:44