A square box of volume V contains N molecules of an ideal gas. Each molecule has mass m. Using the kinetic theory of ideal gases, it can be shown that, if all the molecules are moving with speed v at right angles to one face of the box, the pressure p exerted on the face of the box is given by the expression pV = Nmv². This expression leads to the formula p = (1/3)ρ(c²) for the pressure p of an ideal gas, where ρ is the density of the gas and ⟨c²⟩ is the mean-square speed of the molecules. An ideal gas has volume, pressure and temperature as shown in Fig. 2.1. The mass of the gas is 20.7 g. [Figure 2.1]