Absolutely Last Post on this subject
From Wikipedia
Temperature and pressure
The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:
where ρ is the air density, p is absolute pressure, R is the specific gas constant for dry air, and T is absolute temperature.
The specific gas constant for dry air is 287.05 J/(kg•K) in SI units, and 53.35 (ft•lbf)/(lbm•R) in United States customary and Imperial units.
Therefore:
• At IUPAC standard temperature and pressure (STP) (0 °C and 100 kPa), dry air has a density of 1.2754 kg/m3.
• At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m3.
One cubic meter (m3)of air has a mass of, at STP, 1 m3 X 1.2754 kg/ m3 = 1.2754 kg.
Air is composed of many molecules 20.9% of which are Oxygen or 0.209 x 1.2754 kg = 0.26656 kg of Oxygen.
The mass of one Oxygen atom with 8 neutrons is 2.67803909 x 10-26 , the number of Oxygen molecules in .26656 kg is .26656/( 2.67803909 x 10-26 X 2) = 9.9535 x 1024 molecules.
One cubic meter (m3)of air has a mass of, at 20 °C, 1 m3 X 1.2041 kg/ m3 = 1.2041 kg.
Air is composed of many molecules 20.9% of which are Oxygen or 0.209 x 1.2041 kg = 0.251657 kg of Oxygen.
The mass of one Oxygen atom with 8 neutrons is 2.67803909 x 10-26 , the number of Oxygen molecules in .251657 kg is .251657/( 2.67803909 x 10-26 X 2) = 9.39706 x 1024 molecules.
Subtracting the two gives 9.9535 x 1024 - 9.39706 x 1024 = 5.56441 x 1023 or 5.59% less molecules.