According to Dalton's law of partial pressures, a mixture of gases inflict pressure in a direct proportion to their volume/mass etc. Therefore, since 79 percent of the air is composed of nitrogen and 21 percent of Oxygen, we multiply 101.3 kPa by 0.79 = 80.027 kPa by Nitrogen, 101.3 kPa by 0.21 = 21.273 kPa by Oxygen.
