How much of the air has oxygen?

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Multiple Choice

How much of the air has oxygen?

Explanation:
Air is a mix of gases, and the amount of each gas is described by its volume (mole) fraction. In dry air at sea level, oxygen makes up about 20.95% of the molecules—roughly one out of every five molecules. The rest is mostly nitrogen (about 78%), with small traces of other gases. That’s why the correct figure is about 20.95% for oxygen, not 78%, 0.03%, or 50%. Humidity can slightly reduce the oxygen fraction, but under standard dry-air conditions it’s about 20.95%.

Air is a mix of gases, and the amount of each gas is described by its volume (mole) fraction. In dry air at sea level, oxygen makes up about 20.95% of the molecules—roughly one out of every five molecules. The rest is mostly nitrogen (about 78%), with small traces of other gases. That’s why the correct figure is about 20.95% for oxygen, not 78%, 0.03%, or 50%. Humidity can slightly reduce the oxygen fraction, but under standard dry-air conditions it’s about 20.95%.

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