And so when we talk about things like vapor pressure of a gas, vapor pressure is exactly this. It's the force that these molecules exert on the walls of the container at equilibrium that's vapor pressure. So let's jump into number or part D.
So gas molecules expand into container. And that's because whenever I have this container and does this container out of air, if I open this up, what will happen to gas molecules? Well, remember, gas molecules are moving at high speeds. So if I open something up, they will escape.