Question

What is the oxygen content of dry air in the atmosphere?

Answer 1

See atmosphere of earth on Wikipedia

Explanation:

By volume, dry air typically consists of `78.09%` nitrogen, `20.95%` oxygen, `0.93%` argon, and `0.04%` carbon dixoide. Usually, air also contains small but variable amounts of water vapor, which takes up on average `1%` of volume at sea level.

Except for water vapor, these values stay mostly constant until you reach an altitude of `10` km.

Note however this is for volumes only. To get masses or moles, you can assume that these gases are ideal gases and use the ideal gas law to get somewhat accurate figures.