Chapter 11 – Properties of Solutions 11.5 – 11.8 Notes AP Chemistry

Colligative Properties Properties of a solution that depend only on the number, and not the identity, of the solute particles. Freezing-point depression, boiling-point elevation, and osmotic pressure. Colligative properties are useful for determining the molar mass of different compounds.

Boiling-Point Elevation A nonvolatile solute will lower the vapor pressure of the solvent. A nonvolatile solute elevates the boiling point of the solvent. ∆T = Kbmsolute, where Kb is a constant characteristic of the solvent called the molal boiling-point elevation constant, m is the molality of the solute, and ∆T is the difference between the boiling point of the pure solvent and the solution.

Example A 2.00 g sample of a large biomolecule was dissolved in 15.0 g of carbon tetrachloride. The boiling point of this solution was determined to be 77.85oC. Calculate the molar mass of the biomolecule. For carbon tetrachloride, the boiling-point constant is 5.03oCxkg/mol, and the boiling point of pure carbon tetrachloride is 76.50oC.

Freezing-Point Depression A nonvolatile solute will lower the vapor pressure of the solvent. A nonvolatile solute depresses the freezing point of the solvent. ∆T = Kfmsolute, where Kf is a constant characteristic of the solvent called the molal freezing-point depression constant, m is the molality of the solute, and ∆T is the difference between the freezing point of the pure solvent and the solution.

Example The freezing point of t-butanol is 25.50oC and Kf is 9.1oCxkg/mol. Usually t-butanol absorbs water on exposure to air. If the freezing point of a 10.0 g sample of t-butanol is 24.59oC, how many grams of water are present in the sample?

Osmosis Occurs when a pure solvent and its solution are separated by a semi-permeable membrane that prevents the passage of solute molecules. Solvent molecules will flow into the solution.

Osmotic Pressure Minimum pressure that stops the osmosis is equal to the osmotic pressure of the solution. Π = MRT, where Π is the osmotic pressure, M is the molarity of the solution, R is the ideal gas constant, and T is temperature. Solutions with identical osmotic pressures are said to be isotonic. Reverse osmosis occurs when an external pressure larger than the solution’s osmotic pressure is applied.

Example A 20.0 mg sample of a protein is dissolved in water to make 25.0 mL of solution. The osmotic pressure of the solution is 0.56 torr at 25oC. What is the molar mass of the protein?

Colligative Properties of Electrolyte Solutions Electrolytes dissociate into more than one component. When dealing with an ionic solution the van’t Hoff factor, i, must be used:. i = (moles of particles in solution/ moles of solute dissolved) van’t Hoff factor can be less than expected due to ion pairing in solution.

Example How would you prepare a 1.0 L aqueous solution of sodium chloride having an osmotic pressure of 15 atm at 22oC?

Colloids Suspension of tiny particles in solution. Stabilized due to the electrostatic repulsions between ion layers surrounding the individual particles. A colloid can be coagulated by heating or by adding an electrolyte.