 Collegative properties to vapor pressure and osmotic pressure. Vapor pressure. You can think of vapor pressure as the tendency of something to escape the liquid phase to go into the gas phase. It works similarly as boiling point, but the relationship between them is inversely proportional. That means if the solution has higher vapor pressure, then this own molecule is more likely to escape into the gas phase, which means that the boiling point is lower. When you add solues to the solvent, because it's harder for the solvent molecule to escape into the vapor or gas phase, the vapor pressure decreases. So the vapor pressure of the solution is always less than that of the solvent. So here's the equation. Vapor pressure of solution is equal to the mole fraction of the solvent times the vapor pressure of the solvent. So the mole fraction is just the ratio between the number of moles of the solvent and the total number of moles, which is equal to the sum of the number of moles of the solute and the number of moles of the solvent. Now the number of moles of the solute depends on the Van Gogh factor, right? So if your solute has a Van Gogh factor of 1, like sugar, then adding one mole of sugar effectively means adding one mole of solute. But if your solute has a Van Gogh factor of 2, like sodium chloride, then adding one mole of sodium chloride effectively means adding two moles of solute. Okay, osmotic pressure. What's osmotic pressure? Osmotic pressure is the minimum pressure that stops the inward flow of its pure solvent across a semipermeable membrane. Now let's look at this tube, which has a semipermeable membrane in the middle. Let's put a bunch of water in there. When nothing is happening, the water will diffuse across the membrane and maintain an equal water level on both sides. I know my drawing makes it make the glass seems to have uneven volume on both sides, but let's pretend like it is. Now let's add some salt to the left side. The salt cannot cross the semipermeable membrane, only water, that's why they call it semipermeable membrane, because this is important when talking about osmotic pressure. If your membrane doesn't let anything in or let everything in, the whole osmotic pressure concept won't apply. So when you add the salt in, what will happen to the water level? Now if you guess that some of the water will move to the side with the salt, you are right. The solvent will move to the place with higher solute concentration in an effort to balance the concentration on both sides. This phenomenon is called osmosis, the diffusion of solvent through a semipermeable membrane from a less concentrated solution into a more concentrated one. There are other factors involved, so you won't usually see all the water move into one side. Now we apply some pressure onto the side with the salt. The pressure will push the water onto the other side which does not have the salt. The amount of pressure that is just enough to achieve that is called osmotic pressure. Let's look at the equation for osmotic pressure. So pi is equal to capital M times R times T times I. Pi, this is the capital Pi, is the osmotic pressure. Capital M is molarity, so it's not molarity, it's molarity. R is the gas constant, T is temperature in Kelvin, and I is the Van Haar factor.