If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:3:21
SAP‑7 (EU)
SAP‑7.C (LO)
SAP‑7.C.1 (EK)

Video transcript

in several other videos we have talked about the ideal gas law which tells us that pressure times volume is going to be equal to the number of moles times the ideal gas constant times the temperature measured in kelvin now in all of our studies of the ideal gas law we assumed that the gases that we were dealing with were ideal and now we're going to think a little bit about what does it mean to be ideal and what do real gases how do they vary from actual ideal gases well in order for us to assume that a gas is ideal we assume that its volume the volume that the gas takes up volume of gas is negligible negligible relative to container to container the other thing we assume is that the molecules of the gas don't interact with each other molecules don't interact now in the real world we know that all molecules take up some volume but it could be a reasonable assumption if we're talking about a really huge container and we don't have that high density of molecules in it it's a reasonable assumption that the volume of the gas itself that the molecules themselves are small in volume collectively relative to the container and it's reasonable in many circumstances to assume that the molecules don't interact maybe they don't have strong intermolecular forces once again because they're taking up a small portion of the volume they might even not get close to each other too often and so that's why these are reasonable assumptions and they allow us to say that pv is equal to nrt which is a valuable thing a valuable approximation in most circumstances but in the real world we do know that in actuality the volume that each molecule takes up some volume and that if you add up all the molecules together they're of course going to take up some volume and if there's enough molecules or if the container is small enough we know that the volume of the gas relative to the container won't be negligible we also know that molecules will interact with each other in some way shape or form two molecules can't occupy the same space at the same time so you definitely have some repulsive forces and you have you might have some even for fairly inert molecules you might have some temporary dipoles that get formed some temporary attraction or some temporary repulsion so if you're dealing with a situation where things are less ideal i'm going to make a character of it where the molecules are taking up a significant volume relative to the container you can't say that the volume of the molecules are negligible relative to the container and we assume that they are interacting with each other they're definitely going to repulse each other they can't occupy the same space at the same time but they might attract each other at some points or or repulse each other at other points and so in this situation where we can't make these assumptions we're going to have to modify the ideal gas law
AP® is a registered trademark of the College Board, which has not reviewed this resource.