Dry air is approximately 78% nitrogen, 21% oxygen and 1% argon, by number of molecules. What is the partial pressure of oxygen in a sample of dry air, if atmospheric pressure is 751 torr?

Question

Dry air is approximately 78% nitrogen, 21% oxygen and 1% argon, by number of molecules. What is the partial pressure of oxygen in a sample of dry air, if atmospheric pressure is 751 torr?'

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Answer 1

The partial pressure of each gas is equal to the mole fraction of that gas times the total pressure (Daltons Law)

In this case, the mole fraction is the same as the % by number of molecules, since Avogadro's number relates moles to molecules it is a constant.

mole fraction of O2 = 21% = 0.21

0.21 x 751 torr = 158 torr