2.2 A one litre bulb at room temperature contains hydrogen gas at a pressure of 10 torr. At t 0, a filament of area 0.2 cm is suddenly heated to incandescence. Under these cônditions hydrogen molecules striking the filament are dissociated. Neutral hydrogen atoms that are produced stick to the walls of the bulb on striking. How long is the mean free path for hydrogen molecules at the initial pressure?