13.3 Vapor Pressure Depression and Raoult's Law

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Raoult's Law

Raoult's Law for vapor pressure depression describes how the vapor pressure of an ideal solvent decreases as the amount of dissolved solute increases.  Generally, Raoult's Law is most accurate for dilute solutions that have a large mole fraction of solvent.

PA = XAPA*          XA = mole fraction A          PA* = vapor pressure of pure A

Ultimately, this means that if a solvent has a mole fraction of 0.95, then 95% of the moles in that solution are the solvent and it will have a vapor pressure that is 95% of what the pure solvent would have.  Similarly, if a solvent has a mole fraction of 0.90, then 90% of the moles in that solution are the solvent and it will have a vapor pressure that is 90% of what the pure solvent would have.  The next examples show the calculations for just such aqueous solutions.

Raoult's Law pure water
Raoult's Law mole fraction 0.95
Raoult's Law mole fraction 0.90

In this example we can see that as the amount of solute increases (left to right) the vapor pressure of water decreases.