How many molecules of air in one liter?

Answer 1

So far as I understand it, and in an effort to avoid actually getting tweezers and counting all the molecules, I gather that any molecule can be used with Avagadro's constant, even a mix of them, hence there are 6.02 x 10^23 molecules in a Mol and that would fill 22.4 litres. Therefore the answer is 6.02 x 10^23 / 22.4 = 2.69 x 10^22 molecules per litre at STP.

To check the answer, assuming that the molecular weight of N2 is 28 and the molecular weight of O2 is 32 and they are mixed in the ratio 79:21 then the total weight of a litre of air should be about (((79*28)+(21*32))/100) x (2.69 x 10^22) / (6.02 x 10^23) = 1.28g/litre and I think that given all of the rounding and guesswork involved, that agrees close enough for me with the 1.2kg/m^3 given on Wikipedia (given that there are 1000 litres in a cubic meter).

I'm sure there are more accurate answers to be had, but I don't have one to share.