w6Collegative1

Colligative Properties of Solutions : 

Colligative Properties of Solutions Physical properties of solutions that depend on the quantity (concentration or number of particles) but not the kind of solute particles. Some physical properties of solutions differ in important ways from those of the pure solvent. Pure water freezes at 0°C, but aqueous solutions freeze at lower temperatures. Ethylene glycol is added to the water in radiators of cars as an antifreeze, to lower the freezing point of the solution. It also raises the boiling point of the solution above that of pure water

Colligative Properties : 

Lowering the Vapor Pressure and Raoult's Boiling-Point Elevation Freezing-Point Depression Osmosis Colligative Properties

Slide3: 

Lowering the Vapor Pressure Experiment showing that a solution possesses a lower vapor pressure than does the pure solvent under the same conditions of temperature and pressure The vapor pressure over a liquid is the result of a dynamic equilibrium: The rate at which molecules leave the liquid surface for the gas phase equals the rate at which gas-phase molecules return to the surface of the liquid. A nonvolatile solute added to the liquid reduces the capacity of the solvent molecules to move from the liquid phase to the vapor phase

Raoult's Law: 

There is no change in the rate at which solvent molecules in the gas phase return to the liquid A nonvolatile solute reduces the rate of vaporization of the solvent. The extent to which a nonvolatile solute lowers the vapor pressure is proportional to its concentration. Doubling the concentration of solute doubles its effect. Raoult's Law

Slide5: 

1.0 mol of a nonelectrolyte, such as glucose, produces essentially the same reduction in vapor pressure in a given quantity of water as does 0.5 mol of NaCl, a strong electrolyte. Both solutions have 1.0 mol of particles because 0.5 mol of NaCl dissociates to give 0.5 mol of Na+(aq) and 0.5 mol of Cl-(aq). Raoult's Law: Quantitatively, the vapor pressure of solutions containing nonvolatile solutes is given by P Partial pressure, X mole fraction,

Examples: 

The vapor pressure of water is 17.5 torr at 20°C. Imagine holding the temperature constant while adding glucose, C6H12O6, to the water so that the resulting solution has XH2O = 0.80 and XGlu = 0.20. What is , the vapor pressure of water over the solution = 14 torr Examples

Slide7: 

Glycerin, C3H8O3, is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25°C. Calculate the vapor pressure at 25°C of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure of pure water at 25°C is 23.8 torr

Slide8: 

The vapor pressure of pure water at 110°C is 1070 torr. A solution of ethylene glycol and water has a vapor pressure of 1.00 atm at 110°C. Assuming that Raoult's law is obeyed, what is the mole fraction of ethylene glycol in the solution? Answer: 0.290 P°H2O =1070 torr PH2O = 1 Atm = 760 torr XH2O = --------- PH2O P°H2O = --------- 760 torr 1070 torr = XH2O + XEG = 1 0.7103 + XEG = 1 1- 0.7103 = XEG XEG = = 0.290

Slide9: 

Many solutions do not obey Raoult's law exactly: They are not ideal solutions. If the intermolecular forces between solvent and solute are weaker than those between solvent and solvent and between solute and solute, then the solvent vapor pressure tends to be greater than predicted by Raoult's law. Conversely, when the interactions between solute and solvent are exceptionally strong, as might be the case when hydrogen bonding exists, the solvent vapor pressure is lower than Raoult's law predicts. Although you should be aware that these departures from ideal solution occur, we will ignore them for the remainder of this chapter.

More Examples: 

More Examples Sucrose is a nonvolatile, nonionizing solute in water. Determine the vapor pressure lowering, at 27°C, of a solution of 75.0 grams of sucrose, C12H22O11, dissolved in 180. g of water. The vapor pressure of pure water at 27°C is 26.7 torr. Assume the solution is ideal. Vapor Pressure Lowered = 26.7-26.1= 0.6

Slide11: 

solution is made by mixing 52.1 g of propyl chloride, C3H–Cl, and 38.4 g of propyl bromide, C3H–Br. What is the vapor pressure of propyl chloride in the solution at 25°C? The vapor pressure of pure propyl chloride is 347 torr at 25°C and that of pure propyl bromide is 133 torr at 25°C. Assume that the solution is an ideal solution.

Slide12: 

. At 25°C a solution consists of 0.450 mole of pentane, C”H12, and 0.250 mole of cyclopentane, C”H10. What is the mole fraction of cyclopentane in the vapor that is in equilibrium with this solution? The vapor pressure of the pure liquids at 25°C are 451 torr for pentane and 321 torr for cyclopentane. Assume that the solution is an ideal solution.