The 4 Colligative Properties help to explain what happens to a pure solvent as solute is added to it.
Concept #1: The Colligative Properties.
The four colligative properties of Boiling Point, Freezing Point, Vapor Pressure and Osmotic Pressure will either increase or decrease with the addition of solute to a pure solvent.
Example #1: Explain what happens to each of the following properties as solute is added to a pure solvent.
a. Boiling Point b. Freezing Point
c. Osmotic Pressure d. Vapor Pressure
Example #2: Which of the following compounds will have the highest boiling point?
a) 0.10 m sucrose
b) 0.10 m CsBrO3
c) 0.35 m CH3OH
d) 0.15 m SrBr2
Example #3: Pure water boils at 100oC. What is the expected boiling point of water after the addition of 13.12 g calcium bromide, CaBr2, to 325 g water. Kb = 0.512 oC/m. (MW of CaBr2 is 199.88 g/mol).
Example #4: The vapor pressure of water at 100.0oC is 0.630 atm. Determine the amount (in grams) of aluminum fluoride, AlF3, (in grams) needed to reduce its vapor pressure to 0.550 atm. (MW of AlF3 is 83.98 g/mol).
Practice: Beta-carotene is the most important of the A vitamins. Calculate the molar mass of Beta-carotene if 25.0 mL of a solution containing 9.88 mg of Beta-carotene has an osmotic pressure of 56.16 mmHg at 30 degrees of Celsius.
Vapor Pressure is achieved by the equilibrium rate of vaporization equaling the rate of condensation.
Concept #2: Defining Vapor Pressure.
Example #5: The vapor pressure of pure liquid A is 550 torr and the vapor pressure of pure liquid B is 320 torr at room temperature. If the vapor pressure of a solution containing A and B is 465 torr, what is the mole fraction of A in the solution?
Example #6: Determine the vapor pressure lowering associated with 1.32 m C6H12O6 solution (MW: 180.156 g/mol) at 25oC.
Practice: The following boiling points belong to one of the following compounds: 117°C, 78°C, 34.5°C and 23°C.
a) Which boiling point goes with what compound?
b) If each of the following substances were placed in separate sealed clear bottles at room temperature, could you identify one of the substances right away?