Raoult’s Law

PA = XAP°A

  where

•XA is the mole fraction of compound A

•P°A is the normal vapor pressure of A at that temperature

–

NOTE:  This is one of those times when you want to make sure you have the vapor pressure of the solvent.

•The lowering of the vapour pressure when a non-volatile solute is dissolved in a volatile solvent (A) can be described by Raoult’s Law:

•

  PA = cAP°A

PA = vapour pressure of solvent A above the solution

cA = mole fraction of the solvent A in the solution

P°A = vapour pressure of pure solvent A

Vapor pressure of water is 23.8 torr at 25°C.

 

If you have a solution of water and the nonvolatile solute glycerin at 25°C, where the mole fraction of the glycerin is 0.23, what is the vapor pressure above that solution?

•Since BOTH liquids are volatile and contribute to the vapour, the total vapor pressure can be represented using Dalton’s Law:

•

  PT = PA + PB 

  The vapor pressure from each component follows Raoult’s Law:

  PT = cAP°A + cBP°B

Also,  cA + cB = 1     (since there are 2 components)