Approximately what percentage of dry air is oxygen?

• A. 20.93%
• B. 79%
• C. 0.03%
• D. 50%

Answer: (A)

Oxygen makes up about one-fifth of dry air by volume, with a precise value of 20.93% in standard references. This reflects the mole fraction of O2 in Earth’s atmosphere at sea level when water vapor is not present. So, roughly 21 parts of oxygen for every 100 parts of air, with nitrogen making up most of the rest and only small amounts of other gases. On humid days, water vapor adds more moles to the mix, slightly reducing the oxygen percentage, but the dry-air figure used in many contexts is 20.93%. The other options don’t fit because they either describe nitrogen, an unrealistically high share, or an amount far smaller than oxygen’s actual presence in the air.