11 58 colligative properties let us consider the

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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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This document was uploaded on 02/28/2014 for the course CHEM 311 at LA Tech.

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