Problem 2.227

Oxygen is enclosed at the temperature $0^{\circ} \mathrm{C}$ in a vessel with the characteristic dimension $l=10 \mathrm{~mm}$ (this is the linear dimension determining the character of a physical process in question). Find: (a) the gas pressure below which the mean free path of the molecules $\lambda>l$ (b) the corresponding molecular concentration and the mean distance between the molecules.