**Unformatted text preview: **mponent A can
also be written for B):
P
æP
l sol − l = RT ln è PA ö − RT ln æ PA ö , or
A
A
ø
è ø
P
(5.73)
l sol = l + RT ln æ P A ö .
ç ÷
A
A
è Aø
If the solution behaves ideally (i.e., obeys Raoult’s law), then PA/P*A = xA.
Therefore, we get
l sol = l + RT ln x A .
A
A
For non-ideal solutions, the ratio PA/P*A is called the activity of A, aA. In this case,
we write
l sol = l + RT ln a A .
A
A
As PA approaches P*A, it is clear that the activity approaches unity. This is the
basis for assigning the value of 1 to the activities of all pure substances.
See problem 5.34.
11 5.8. Colligative Properties
Let us consider the consequence of dissolving a nonvolatile solute B in a solvent
A. Since the solute cannot be present in the vapor phase, this immediately leads to
a lowering of vapor pressure, because xA < 1 and, therefore, PA = xAP*A < P*A.
This lowering of vapor pressure is responsible for both lowering the freezing point
as well as raising the boiling point of the solvent.
Freezing Point Depression:
For the equilibrium between the liqui...

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