notes - Unit: Physical Properties of Solutions Module:...

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Unit: Physical Properties of Solutions Module: Colligative Properties [page 1 of 1] Vapor Pressure Lowering www.thinkwell.com info@thinkwell.com Copyright 2001, Thinkwell Corp. All Rights Reserved. 1508.doc –rev 06/20/2001 A colligative property is one that depends on the concentration of the solute but not on the nature of the solute. Raoult’s law states that the vapor pressure of the solvent decreases linearly as the mole fraction of the solute increases. Raoult’s law states that as solute is added to a solution the vapor pressure of the solvent decreases. In an ideal solution , solute molecules are completely free to move around and disperse, which increases the entropy of the system. If molecules of the solvent move into the gas phase, the volume of the solvent is reduced, and therefore the entropy of the solution decreases. This decrease in entropy stabilizes the liquid phase. Vapor pressure decreases as a result. Using Raoult’s law, a change in the vapor pressure of 78.0 g of benzene after 6.40 g of naphthalene is dissolved can be calculated from the mole fraction of the solvent and its original vapor pressure (0.1252 atm). First, the moles of benzene and moles of naphthalene must be calculated in order to obtain the mole fraction of benzene. The mole fraction of the solvent and its original vapor pressure are then plugged into Raoult’s law to give the new vapor pressure. When there are significant attractions between solute and solvent, H is negative (exothermic), and the liquid phase is further stabilized. This means more energy is required to go from liquid to vapor. The vapor pressure will be lower than that expected for an ideal solution. If the addition of solute causes more bonds between solvent molecules to be disrupted, then H will be positive (endothermic). The formation of solution costs energy, and therefore less energy is required to convert solvent molecules from liquid to vapor. The vapor pressure will be higher than that expected for an ideal solution.
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Unit: Physical Properties of Solutions Module: Colligative Properties [page 1 of 1] Boiling Point Elevation and Freezing Point Depression www.thinkwell.com info@thinkwell.com Copyright 2001, Thinkwell Corp. All Rights Reserved. 1510.doc –rev 06/20/2001 As solute is added to a solvent , the vapor pressure of the solvent decreases, increasing the boiling point of the solution .
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This note was uploaded on 12/09/2011 for the course CHEM 1 taught by Professor Staff during the Summer '11 term at Simon Fraser.

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notes - Unit: Physical Properties of Solutions Module:...

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