Re: Why equal number of molecules of two different gases occupy same volume?

I don't have the text you mentioned, but the statement " because they have 
the same kinetic energy, the same pressure is created" can be misleading 
in trying to understand Avogadro's Law.

First let's review what influences the volume of a gas. Basically four 
factors:

(1) Atmospheric pressure, or the weight per unit area acting on the gas, 
which will influence to what extent the gas can expand against this force.
(2) The number of molecules of the gas. If all three other factors are 
equal, more molecules occupy more volume.
(3) The temperature or average kinetic energy of the molecules. Increasing 
the energy will increase volume, provided that, again, all other factors 
are equal.
(4) Intermolecular attractions between the molecules can decrease the 
volume expected from only considering temperature, pressure and number of 
molecules.

Now consider two cylinders of gases of different masses, each free to push 
up against a piston. Each has the same number of molecules, subjected to 
the same atmospheric pressure. If they are at the same temperature, the 
smaller molecules will, as you pointed out, move faster. But this does not 
change the external pressure acting on each piston. And although the 
smaller molecules are moving faster, at the same temperature the larger 
molecules carry the same energy. So basically you have two different 
systems pushing against the same force with the same energy. Thus, if 
intermolecular forces are equal, Avogadro's Law makes theoretical sense: 
the gases assume the same volume.

Enrico Uva