1. Colligative Properties Test Chapter 15 Tuesday May 7, 2002

2. Colligative Properties Colligative Properties are those properties of a liquid that may be altered by the presence of a solute. Examples of these properties are: the vapor pressure the freezing and boiling points and the osmotic pressure. All of these properties ultimately relate to the vapor pressure.

3. The vapor pressure of a solvent depends on how pure it is.

4. Raoult’s Law P solvent = X solvent P o solvent Pure solvent Mole fraction Solution

5. Boiling Point Elevation Pure water boils at 100ºC A solution of water and sucrose boils at MORE than 100 ºC The more sucrose we add the HIGHER the boiling point.

6. Freezing Point Depression Pure water freezes at 0ºC A solution of water and sucrose freezes at LESS than 0ºC The more sucrose we add the LOWER the freezing point.

7. Impurities in a substance cause a change in its phase diagram by making the liquid region bigger The addition of solute RAISES the boiling point and LOWERS the freezing point of a solvent.

8. MOLALITY Molarity (M) number of moles per liter of solution MOLALITY (m) number of moles per kilogram of solvent. number of moles m = ---------------------------------- mass of solvent (kg)

9. Molality: Problem 1 You pour 12 g of KBr into a beaker that contains 600 mL of water. What will be the molality of the resulting solution? M KBr = 39.098 + 79.904 = 119.002 g n = 12/119.002 = 0.10084 mol KBr mass (H 2 O) = 600 mL = 600 g = 0.60 kg m = 0.10084 mol / 0.60 kg = 0.168 m

10. Molality: Problem 2 How many grams of MgF 2 would you need to prepare a 0.75m solution of MgF 2 using 220g of water? M MgF2 = 24.305 + 2 x 18.996 = 62.297 g mass of Mg = 0.75m x 0.220kg x 62.297 g = 10.2790 g

11. Colligative Properties: Computations Boiling Point elevation: ΔT b = ٭ k b m Molality Coefficient of BP elevation Number of particles Change in BP (ºC) etiin

12. Colligative Properties: Computations Freezing Point depression: ΔT f = ٭ k f m Molality Coefficient of FP depression Number of particles Change in FP (ºC) etiin

13. Number of particles in solution When sucrose is dissolved in water, the molecules remain as one particle. C 12 H 24 O 12 (s)  C 12 H 24 O 12 (aq) When sodium chloride is dissolved in water, it dissociates into 2 particles: NaCl(s)  Na + (aq) + Cl – (aq) When calcium chloride is dissolved in water, it dissociates into 3 particles CaCl 2 (s)  Ca + + 2 Cl – (aq) NaCl and CaCl 2 are called electrolytes because their solution conduct electricity.

14. The larger the number of particles released by a solute the greater its effect on the BP and FP. So we would expect: NaCl to be twice as effective as sucrose CaCl 2 to be thrice as effective as sucrose The size of the particles is unimportant In real life ionic compounds do not dissociate completely

16. Boltzmann curve for pure solvent

18. OSMOSIS is the movement of solvent through a membrane to equalize the concentration on both sides.