A low-pressure weather system comes into the city of Denver. The atmospheric pressure is 715 mmHg. If 78.0% of dry air is nitrogen, what's the partial pressure of nitrogen in low-pressure system?

Answer 1

The partial pressure of N2 in air is proportional to the mole fraction of N2 in the air. As dry air is primarily a mixture of N2 and O2, the mole fraction can be determined from the density of dry air.  At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m³; so the mass of N2 in dry air is 0.78 x 1.204 kg /m³ = 0.939 kg /m³, and the mass of O2 is 0.22 x 1.204 kg /m³ = 0.265 kg /m³.

The mole fraction of N2 in dry air is approximately

(0.939 /28) / (0.939/28 + 0.265 /32) = 0.802.

As the total pressure is 715 mm Hg, the partial pressure of N2 is 0.802 x 715 = 573 mm Hg

Keep in mind, this assumes the density given the particular temperature and pressure stated. Also, the contribution of CO2 and other minor gases was neglected.