Colligative Properties and Raoult's Law

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  • 0:06 Colligative Property…
  • 1:13 Raoult's Law
  • 2:01 Examples
  • 3:40 Osmotic Pressure
  • 4:45 Determining Molar Mass
  • 6:43 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn how vapor pressure and osmotic pressure are colligative properties. Learn Raoult's Law and how to use it to determine the vapor pressure of a solution. Learn the equation for determining osmotic pressure and how to use it to determine the molar mass of a substance.

Colligative Property and Vapor Pressure

You have previously learned that a colligative property is a property that depends on the concentration of solute particles but not on the identity of the solute. You know that freezing point depression and boiling point elevation are colligative properties. The vapor pressure of a solution is also a colligative property. Vapor pressure is the pressure of the vapor over a liquid at equilibrium. A nonvolatile solute will lower the vapor pressure of a solvent.

What this means is that adding a solute to a solution will lower the pressure of the gas over the liquid. This makes sense when you think about it. By adding a solute to a solvent, we are diluting the solvent. Now it is diluted and there are fewer molecules at the surface, so it is easier to escape.

You can think of it like a classroom with only one door. When the class is small, say only 10 students, it is easy to leave when the bell rings. When the class is huge, 35 or more students, it gets much harder to get out the door. It is the same idea with solute particles and leaving the surface of a liquid.

Raoult's Law

Francois Raoult developed the equation to find the vapor pressure of a solution.
Francois Raoult

In the late 1800's, Francois Raoult carried out extensive studies on vapor pressure, and he came up with an equation for them that is called Raoult's Law. His equation is:

Psolution = Xsolvent * P°solvent

Psolution = the observed vapor pressure of the solution

Xsolvent = the mole fraction of the solvent

solvent = the vapor pressure of the pure solvent

This equation allows you to find the vapor pressure of a solution when you have the other information.

It also helps scientists determine the number of moles of solute present and the molar mass. The vapor pressure also allows scientists to characterize solutions.


What is the vapor pressure for a solution that has 200 g of sugar (C12 H22 O11) in 700 g of water? Water's vapor pressure is 23.76 torr, its molar mass is 18 g/mole and sugar's molar mass is 342.3 g/mole.

First, you have to determine how many moles of sugar you have.

200 g sugar x 1 mole sugar/342.3 g = 0.58 moles sugar

Now, you need to determine how many moles of water you have.

700 g H2 O x 1 mole H2 O/18 g = 38.9 moles H2 O

Now, determine the mole fraction of the solvent, water, in the solution.

X solvent = moles solvent / moles solvent + moles solute

X H2 O = 38.9 moles H2 O / 38.9 moles + 0.58 moles = 0.985

Now, you have all the information you need to use Raoult's equation.

Psolution = Xsolvent *P°solvent

Psolution = 0.985 (23.76 torr)

Psolution = 23.41 torr

So, adding the sugar to the water lowered its vapor pressure from 23.76 torr to 23.41 torr.

Osmotic Pressure

Osmotic pressure is another colligative property. To understand osmotic pressure, let's look at the vocabulary involved. Osmosis is the flow of a solvent between solutions separated by a semipermeable membrane. Osmotic pressure is the minimum pressure that must be applied to a solution to stop osmosis from happening.

Osmotic pressure can be determined with the formula II=iMRT.

II = the osmotic pressure measured in atmospheres

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