Colligative Properties

Vapour pressure and Raolt´s law

Pure water

The vapour pressure is caused by some molecules, turning from liquid to gas and therefore causing vapour pressure.

Solution of pure water and salt (that is not volatile, won’t turn into gas)

Because there are now salt molecueles taking up the space on the surface,less water molecules can turn to gas. So vapour pressure is lower.

So the more solute that is added, the lower the vapour pressure.

Raolt discovered that the…

Vapour pressure of a solution (P)= vapour pressure of the pure solvent  (Po)  x   molar fraction of the pure solvent  (Xo)         

This is called Raotts´s Law. It can only be used when we make a solution with a solute that is non-volatile so will not turn to a gas.

At 25C, water has a vapour pressure of 3,2 kpa. If I make a solution with 5 moles of sugar and 55 moles water, what is the new vapour pressure?