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