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Visit A-Level Chemistry to download comprehensive revision materials - for UK or international students! The pressure at which vapor is formed above a solid or liquid at a particular temperature is called the vapor pressure.
Vapor and solid or liquid are in dynamic equilibrium at this temperature. In s Rault discovered when a non-volatile solute is dissolved in a solvent the vapor pressure decreases.
The lowering of vapor pressure depends on the mole fraction of the solute dissolved and the vapor pressure of the pure solvent. If P is the vapor pressure of pure solvent and P s is the vapor pressure of the solution.
The lowering of vapor pressure is P — P s. The vapor pressure depends on the number of molecules evaporate from the surface.
If non volatile solute dissolved in a solvent the non volatile molecule blocks the fraction of solvent molecules to evaporate. Thus as the less number of molecules evaporate from the surface the vapor pressure also lesser than the pure solvent.
This vapor pressure of the solution can be determined by the number of solvent molecules present on the surface or the mole fraction of solvent.
Assume w grams of solute dissolved in W grams of solvent.
M and m is the molecular weight of solvent and solute. Thus the equation can be rewritten as.
What is the Importance of Raoult’s law?
Suppose A and B are the molecule of solvent and solute. Thus it is an idea solution.
As in very dilute solution the molecular size and attraction of solute and solvent are similar. Rate this note. Explanation If P is the vapor pressure of pure solvent and P s is the vapor pressure of the solution.