Lecture_5b

73 l sol l rt ln p a a a a if the solution behaves

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Unformatted text preview: um with their vapors, we write P P l  = l ,vap + RT ln æ PA ö and l  = l ,vap + RT ln æ PB ö B B A A è ø è ø (5.72) From the last two equations, we write (all equations written for component 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 5.32. 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...
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This document was uploaded on 02/28/2014 for the course CHEM 311 at LA Tech.

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