The Chemistry of Liquids, Part 5- Dalton's Law of Partial Pressures and Raoult's Law

Dalton's Law of Partial Pressures

John Dalton, a brilliant scientist during the late 1800's, discovered that the total pressure of a mixture of gases is equal to the sum of their partial pressures (the pressure each would have alone).  The assumption is made that both gases are ideal, meaning that any effects of interactions between them are negligible. 

Raoult's Law

Raoult's Law tells us that the partial vapor pressure of a liquid, among a mixture of volatile liquids, is directly proportional to its mole fraction.  The mole fraction is determined by taking the number of moles of a liquid and dividing it by the total number of moles.  Here is the Raoult's Law formula:
Px = X * Po (In words: The partial vapor pressure is equal to the solvent mole fraction times the vapor pressure of the pure solvent.)

 

Illustration of Dalton's and Raoult's Laws, Combined

The following drawing shows an example for a mixture of two volatile liquids: 1,2,3-trichlorobenzene and bromobenzene.  The solvent vapor pressures were obtained from "CRC Handbook of Chemistry and Physics, 61st Edition, 1980-1981".

Raoult's law is first applied to obtain the partial vapor pressure for each solvent of the mixture.  Then, Dalton's Law of Partial Pressures is used to calculate the total vapor pressure.  The green color of the mixture container is meant to indicate the combination of the yellow and blue pure solvent containers.  The mole fraction values were arbitrarily chosen.

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