Colligative+Properties

=**Colligative Properties**= The properties of a solvent that depend on the total concentration of solute particles present. For example, boiling and freezing points change depending on the concentration of other substances within the solute. Aqueous solutions normally have freezing points that are lower that zero degrees Celsius. The different factors that change depending on concentration are vapor pressure, boiling point and freezing point.

Vapor Pressure:
When an nonvolatile solute is added to a solvent the vapor pressure of the solution is lower than that of the initial solvent.



This picture shows that adding a nonvolatile solute to a solution lowers the vapor pressure.

Raoult's Law: the partial pressure exerted by solvent vapor above a solution equals the product of the mole fraction of the solvent in the solution multiplied by the vapor pressure of the pure solvent. Raoult's Law is the law used for an ideal solution, just as the ideal gas law is used for ideal gases. Solutions follow ideal behavior when the solute concentration is low and the solute and solvent have similar molecular sizes and similar intermolecular attractions. There are many solutions that do not obey Raoult's law. The pressure predicted by Raoult's law tends to be greater when the intermolecular forces between the solvent and solute are weaker than the forces in solvent solvent and solute solute. Partial pressure exerted by the solvent vapor equals the product of the mole fraction of the solvent times vapor pressure of the pure solvent.

**Boiling Point Elevation**
Remember that normal boiling point is the temperature where the vapor pressure equals 1 atm. Adding a solute to a solvent lowers the vapor pressure so a higher temperature is required to boil the solution. //The boiling point of the solution is higher than that of the pure liquid//

K is the molal boiling point elevation constant, which depends on the solvent.

Freezing Point
Freezing-Point DepressionThe freezing point of a pure liquid is located at the liquid’s triple point. Because the vapor pressure of a solution is lower than the vapor pressure of a pure liquid, the triple point temperature of a solution is lower than that of a pure liquid. Therefore, the freezing point of a solution is lower that the freezing point of a pure liquid. The decrease in the freezing point of a solution is represented by the equation:

∆Kf is the molal freezing-point-depression constant, which represents degrees Celsius per molarity.

Delta T is the decrease in boiling point and K is the molal freezing point depression constant.

Osmosis
The process of osmosis occurs when solvent molecules move through a semi-permeable membrane from a less concentrated solution to a more concentrated solution. The semi-permeable membrane allows some molecules, like the small molecules of a solvent to pass through, but does not allow larger molecules to pass. Between two solvents of different concentrations, the net movement of solvent will always be to the solution with a higher concentration. After awhile the osmosis process stops, this occurs when the more highly concentrated solution’s pressure on the membrane becomes so great that the effects of osmosis are stopped. The pressure required to stop osmosis is called the osmotic pressure. Osmotic pressure follows a rule very similar to PV=nRT:

However in this equation M represents the molarity of the solution.

Isotonic solutions are two solutions with equal osmotic pressure, so no osmosis occurs. A hypotonic solution is the solution with the lower concentration or osmotic pressure. The hypertonic solution is the solution with the higher concentrate. Osmosis is a very important process and essential for living. The membranes of red blood cells are semi-permeable and they allow water to pass through, but prevent ionic and macromolecular molecules from passing into the cells.



Works Cited: Chemistry: The Central Science Brown, LeMay, Bursten

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