Colligative Properties

Vapor pressure is caused by some molecules turning from a liquid into a gas.

(dibujo)
 Pure Water  
(dibujo)                                
solution of pre water and salt (not volatile)
Because there are now salt molecules taking up the spare surface, less molecules can turn to gas. So vapor pressure is lower.

knowing this we can assure that when there is more solute added to the solution, the vapor pressure will decrease.
Francois-Maire Raolt discovered that:

Vapor pressure of solution = vapor pressure of solvent x molar fraction of the solvent
              (P)                                         (Po)                                      (Xo)
This is called Raolt's Law. It can only be used when we make a solution with a solute that is non-volatile (which means it will not turn to gas).

Problem:
At 35 ºC water has a vapor pressure of 3.2 kPa (finish)