# Is pressure equalization "diffusion"?

There is another question on the site which is:

Two containers, A and B, connected by a closed valve were evacuated and the Chlorine gas (Cl2) with a pressure of 200kPa was put into A and Oxygen (O2) with a pressure of 150kPa into B. a)If Container A has a volume of 10L and container B has a volume of 5L, find the total pressure within the system if the valve were opened and the gases allowed to mix b)Which of the gases would diffuse through the valve at a faster rate?

It has been a long time since I took chemistry and blundering around with Google didn't really seem to answer my question about a technical point about question (b) for this problem.

Obviously when you first open the valve between containers A and B the pressure is going to equalize. Since A has a higher pressure, gas will flow from A into B. Is this diffusion?

In a weird way the pressure equalization does fit the basic notion of diffusion - molecules mover from an area of higher concentration to an area of lower concentration. Thus if both containers had oxygen, then container A would be more concentrated (e.g mol $\rm L^{-1}$) since it has a higher pressure. Thus the molecules will move from A to B because there is a lower "concentration." However this doesn't really seem to fit my notion of either diffusion or effusion. So is there a better proper term for this or is the phrase "pressure equalization" about as good as can be done? Or am I just wrong?

Obviously after the pressures equalize, then the oxygen from the oxygen/chlorine mixture will diffuse into the chlorine faster since oxygen molecules have a lower mass.

The gist is that the question would be simple to answer if containers A and B had the same pressure. But the difference in pressure seems to make the question turn on a very exacting definition of diffusion.