Vapor Pressure Lowering
Vapor pressure, the pressure of the evaporated solvent, decreases as the concentration of solute particles increases. This happens because the solute particles take up a portion of the surface area between the solvent and the gas it is evaporating into. Since the solvent has less opportunities to evaporate there are less evaporated or vaporized solvent particles, thus a decreased vapor pressure.
For an ideal solution, the equilibrium vapor pressure is given by Raoult's law as
- , where
is the vapor pressure of the pure component i (= A, B, ...) and is the mole fraction of the component i in the solution
For a solution with a solvent (A) and one non-volatile solute (B), and
The vapor pressure lowering relative to pure solvent is, which is proportional to the mole fraction of solute.
If the solute dissociates in solution, then the vapor pressure lowering is increased by the van't Hoff factor, which represents the true number of solute particles for each formula unit. For example, the strong electrolyte MgCl2 dissociates into one Mg2+ ion and two Cl- ions, so that if ionization is complete, i = 3 and . The measured colligative properties show that i is somewhat less than 3 due to ion association.
Read more about this topic: Colligative Properties
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