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When we say that nitrogen occupies 78% , oxygen 21% and other gases 1% by volume of the total volume of air, what do we actually mean by it?

I am confused because I have been told that molecules of gases occupy all space given to them therefore molecules of nitrogen must ocuppy all space given to them in earth's atmosphere and in that cases it's volume percentage will be 100%( all volume occupied of given volume). I know that there is some fault but where it is?

I think that when we say thay nitrogen occupies 78% by volume of air then we mean that if we keep all of earth's atmosphere in a box and seperate all nitrogen molecules on one side than the space they would now occupy would be 78% of the space of the box?

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    $\begingroup$ Yes,78% is the percentage of nitrogen by volume. Gas compositions are frequently reported by volume because the ideal gas law is assumed meaning that all gas molecules occupy the same volume. Also note however that the gas composition values by mass would be different. $\endgroup$ – MaxW May 11 at 18:23
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To make $10$ litres air at ordinary pressure and temperature, you will have to fill a syringe with $7.8$ liters nitrogen at ordinary pressure and temperature. You take another syringe with $2.1 $ liters oxygen at ordinary pressure and temperature, then a third string with $0.1$ liter of other gas. Then you mix all these gases by sending them in a fourth syringe, empty at the beginning.

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  • $\begingroup$ Is volume additive that is if we mix 2l of oxygen and 2l on nitrogen in a 2l vessel how can toatal voulme be 4l $\endgroup$ – Kanchan tolani May 15 at 18:44
  • $\begingroup$ Yes. The volumes of gas are additives. The volumes of liquids are not additives. $50$ mL ethanol plus $50$ mL water makes a total of $96$ mL. $\endgroup$ – Maurice May 15 at 20:38
  • $\begingroup$ Therefore according to you mixing 50 ml.of o2 and 50 ml of co2 in an 50 L vessel would give 100 ml total therefore 100 ml in 50 ml $\endgroup$ – Kanchan tolani May 23 at 5:45
  • $\begingroup$ @Kanchen tolani. Volume of gases are additive if the temperature and the pressure of the gases are maintained constant. $\endgroup$ – Maurice May 23 at 9:44

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