At STP, one MOLE of a gas ( 6 x 10^{23} atoms/molecules) occupies 22.4 litres (22,400 cc) = 1366 CI.... One mole of a gas has a weight in grams that is exactly it's atomic/molecular weight.... Nitrogen (N2), weighs (14x2) = 28 MW, and Oxygen (O2) weighs (16x2) = 32 MW.... Air, which is a mixture, is about MW = 29.... However, it has a bit less density than that would predict.... Air has a density at STP of 1.225 g/L.... which works out to 0.31 grains per CI at 1 bar.... Helium weighs 4 AW, so 4 grams for 1366 CI.... which is (4 x 15.43 / 1366) = 0.045 grains per CI at 1 bar.... Air, at STP, is 6.9 times as heavy as He....

If the pressure is 100 bar, then the weight per CI is 100 times greater.... Air would weigh 31 grains/CI, and Helium would weigh 4.5 grains/CI.... At very high pressures, we need to take into account the VanDerWaals correction, of course, because pressure goes up faster than density....

Bob