Dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases
Which means if I were to take a differential volume dV of air, at any instant of time, then 78% of it would be occupied by N2, 21% by oxygen, 0.93% by Ar and so on.
My question is why aren't we considering the empty space? Some volume in that dV volume, will be empty, occupied by nothing. Then why some % is not stated as empty.
Best Answer
It's the percentage by moles, not the percentage by absolute volume. Of all the matter inside the volume that you considered, nitrogen atoms would make 78.08% of the number of atoms and so on. Since the volume of a gas at a particular temperature and pressure is proportional to the number of moles of the gas according to Avogadro's law, the percentage in terms of volume is also an equivalent representation.
But if you take a differential volume $dV$, it's improbable that you'll find any atoms in the volume. Gases have very low density, and you'll be lucky to find even a few atoms inside your differential volume. So the composition would definitely not look like the one you mentioned.