Question: If there is 3 meter cubed of gas how much meter cubed of oxygen, nitrogen and helium are there?

Katherine Manfred answered on 6 Nov 2020:

Hello!

I think we possibly have a little confusion here about gas vs. air? “Air” is a specific mixture of gases that contains nitrogen, oxygen, helium, and thousands of other gases. There is some variability depending on where you are, but typically 78% of air is nitrogen. So, 78% of 3 m^3 is (checks calculator) 2.34 m^3. You can do the same thing for oxygen (about 21% of air).

There is not very much helium present in air. Typically, we measure about 5 ppm, or “parts per million”. When we talk about nitrogen as 78% of air, we’re basically saying that there are, on average, 78 molecules of nitrogen for every 100 molecules of air. Similarly, 5 ppm of helium means that, on average, there will be 5 helium atoms for every 1,000,000 molecules of air! If we do that calculation, we find that 0.000015 m^3 – or, more sensibly, 15 cm^3 – of a 3 m^3 sample of air would be made up of helium atoms.