Quote from Thermactor:
That flies in the face of every chemistry class I have ever taken at my university. I thought that PV=nRT
Sure it seems counter-intuitive at first, but if you consider that any gas a particular temperature will condense at a certain pressure, why would forcing in more gas increase the pressure when it is past the critical point for condensing? Wouldn't it rather just condense?
The equation PV=nRT still holds true, it's just that the pressure is constant while the volume (of the gas) decreases as the liquid level increases and the number of moles of gas decreases (as it becomes liquid).
http://www.vias.org/genchem/phase_equi_12596_05.html
Mixtures of gases is more complicated, as it depends on the solubility of the other gases in the first one that condenses.