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Unformatted text preview: apor pressure of pure A as PA* and that of B as PB*.
Raoult’s law can now be written as
PA = xAPA* and PB = xBPB*.
In other words, the partial pressure of a substance in the vapor phase is determined
by its mole fraction in the liquid phase. Compare this to the Dalton’s law of partial
pressures, which states
PA = yAP and PB = yBP,
where the total pressure is denoted as P. If only A and B are present in the gas
phase above the solution, we have
P = xAPA* + xBPB*.
Since A and B are the only substances present in solution, we may re-write this as
P = xAPA* + (1 – xA)PB*,
which gives the variation of total pressure as a function of xA
Problems 5.19, 5.21, 5.28.
. 4 Henry’s Law:
Henry’s Law may be written in terms of molality, molarity or mole fractions as
PB = kBmB ≈ kB″cB ≈ kB′xB.
In the limit of infinite dilution, solvents usually follow Raoult’s law and solutes
usually follow Henry’s law.
The form of Henry’s law preferred by the authors of the textbook is:
PB = kB′xB, (5.28) where B refers to the component present in small amounts in the solution.
One place w...
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