Natural composition of oxygen in Earth's atmosphere is approximately what percentage?

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Multiple Choice

Natural composition of oxygen in Earth's atmosphere is approximately what percentage?

Knowing how Earth's atmosphere is composed by percent is what this question tests. Oxygen makes up about 21 percent of the air by volume (mole fraction), with nitrogen around 78 percent and the remaining gases totaling the rest in trace amounts. So 21 percent is the best match for the natural air we breathe. The 78 percent option describes nitrogen, not oxygen. The 0.03 percent figure is far too small for the bulk of the atmosphere (trace gases are only a tiny portion). And 100 percent would be pure oxygen, not atmospheric air.

Knowing how Earth's atmosphere is composed by percent is what this question tests. Oxygen makes up about 21 percent of the air by volume (mole fraction), with nitrogen around 78 percent and the remaining gases totaling the rest in trace amounts. So 21 percent is the best match for the natural air we breathe. The 78 percent option describes nitrogen, not oxygen. The 0.03 percent figure is far too small for the bulk of the atmosphere (trace gases are only a tiny portion). And 100 percent would be pure oxygen, not atmospheric air.

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