# How to calculate the average molar mass of the atmosphere? [closed]

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.

## closed as off-topic by David K, Servaes, mechanodroid, José Carlos Santos, Michael GrantOct 19 '17 at 16:37

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question is not about mathematics, within the scope defined in the help center." – David K, Servaes, mechanodroid, José Carlos Santos, Michael Grant
If this question can be reworded to fit the rules in the help center, please edit the question.

• 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. – Sibelephant Oct 19 '17 at 14:34
• It seems like a question for the chemistry stackexchange, keeping in mind their policies with regard to homework: chemistry.meta.stackexchange.com/questions/141/… – 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. – Sibelephant 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 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. – 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.