Example 1 - At the surface of Earth (P= 1 bar and T = 270K, M = 0.0289 kg/mole) what is r?

P=r/M RT : r= mass/volume, M=mass /mole and R = R = gas constant = N_A k = 8.31 J K^-1 mol^-1

r= P M / (RT) = 10⁵ x 0.0289 / (8.3 x 270) = 1.25 kg m^-3 - what's this like? Remember that the density of water is 1000 kg m^-3