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      Solutions and Colligative Properties

The mole fraction of a substance in a mixture is the number of moles of that substance divided by the total number of moles of all substances present:

If a nonvolatile solute, A, is added to a pure solvent, B, until the mole fraction of the original solvent has decreased from one to XB, then the vapor pressure will be only XB times the vapor pressure of the pure liquid, p°B

pB = XBB

This is Raoult's law. The lowering of vapor pressure of B will be proportional to the mole fraction of the added solute, A:


You should be able to show that this follows from the previous expression, with the added fact that the sum of mole fractions is unity:

XA + XB =1

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