Make sure you understand this section, including the meaning of the van der Waal's constants a and b . Gas molecules are indeed in constant, rapid motion. The mixing of two gases is called diffusion . The escape of a gas into a vacuum through a tiny orifice is called effusion . There is an important difference between the two terms.
Diffusion involves the collision of gas molecules.
Thus, although a gas may move at a thousand miles per hour, it may take many minutes for it to move across a room. Effusion is different, since there are no molecular collisions after the molecule escapes from the container.