How do we measure air?

The gases making up air can be separated by physical means. When a lighted candle is allowed to burn in a specified amount of air, part of the air is used up during the burning. The part of air which is used up during burning is oxygen. The other part which does not support burning is largely nitrogen.

Just doing this simple task can tell you the percentage of the air that was oxygen and mostly nitrogen.

We measure it via a process called distillation. Distillation is a fancy word for sorting flavors of a blend by their condensation point.

If you cool a parcel of air to temperatures so cold that oxygen and nitrogen condense and become liquid, at each dew point temperature of each flavor of gas, the flavor of gas condenses to liquid. This is how we obtain liquid oxygen and nitrogen, this is how we obtain pure oxygen and nitrogen. This is how we extract any flavor of gas from our air.

Water vapor is the easiest to extract as liquid water. You see this happen all the time when you place a chilled object outside on a humid day.

you just take a sample of air jar open lid wow you caught air not to hard was it put the lid on and jarred air then you analyse the gases in the jar and repeat then you take the average of all then percent it by number of gas A particles/total gas particles but remember always room for error so those percents are close but probably not dead on

Most of it is done my chemicals. You can actually capture air and its composer and exam it. as of how I dont know