Chemisty homework help please~?

Question

Dry air is approximately 78% nigtogen, 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?

Answer 1

Remember, all ideal gas molecules exert the same pressure in the same situation, so the partial pressure of oxygen is the same fraction of the total pressure as oxygen molecules are of the total molecules. Since 21% of the molecules are O2, 21% of the pressure comes from O2. So the partial pressure is 0.21(751) or 157.71 torr.