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.

  • 2
    $\begingroup$ This is certainly not linear algebra. It looks like chemistry rather than math. $\endgroup$ – David K Oct 19 '17 at 14:33
  • $\begingroup$ I actually didnt know in what section I should put this question. $\endgroup$ – SAINT Oct 19 '17 at 14:34
  • $\begingroup$ It seems like a question for the chemistry stackexchange, keeping in mind their policies with regard to homework: chemistry.meta.stackexchange.com/questions/141/… $\endgroup$ – David K Oct 19 '17 at 14:42
  • $\begingroup$ Thanks for the advise. But I asked the question here, because I actually has this task in my math class. $\endgroup$ – SAINT Oct 19 '17 at 14:47
  • $\begingroup$ I think, that it is ok to ask it on a math board. In the same fashion as this is not on woodworking.stackexchange, this is not on cooking.stackexchange, and this is not on music.stack-exchange. But I don't know the best choices of tags myself. A homework tag would be nice. $\endgroup$ – P. Siehr Oct 19 '17 at 14:48

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.

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

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