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Colligative Properties Notes I. What is a Colligative Property? - A colligative property is a property that is dependent upon the amount of solute (concentration) but not upon the solute identity. *When calculating, you will be using molality (m). II. There are 4 colligative properties: 1) Vapor Pressure Lowering - The boiling point and freezing point of a solution are DIFFERENT than that of a pure solvent! - A nonvolatile substance is one that has little tendency to become a gas under existing conditions. *Look at the graph on page 446 and compare the freezing/boiling points of the solution/solvent This leads us to our next two colligative properties: 2) Freezing Point Depression (Dtf) - The difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent. - Can be calculated with the following calculation: o Dtf = Kf X m X i The variables are: Dtf = the change in the freezing point temperature Kf = the molal freezing point constant (p. 432) m = the molality of the solution i = the number of ions present (covalent = 1, ionic = total number of ions) Example Problem 1: Calculate the freezing point depression of adding 150 g of NaCl into 250 g of water. m = 150g/ 58.44 g/mol / .250 kg= 10.27 m Dtf = Kf X m X i = (-1.86oC/m) (10.27 m) (2) = -38.20 oC = Since the normal freezing point of water is 0oC, we add the -38.16 oC and get: = -38.20 oC Example Problem 2: Calculate the freezing point depression of adding 100 g of CH3OH into 500 g of the non- electrolyte, camphor. m = 100g/ 32.05 g/mol / .500 kg = 6.24 m Dtf = Kf X m X i o = (-39.7 C/m) (6.24 m) (1) o = -247.73 C o o = Since the normal freezing point of camphor is 178.8 C, we add the -247.73 C: o = -68.93 C 3) Boiling Point Elevation (Dtb) - The difference between the boiling points of the pure solvent and a solution of a nonelectrolyte in that solvent. - Can be calculated with the following calculation: o Dtb = Kb X m X i The variables are: Dtb = the change in the boiling point temperature Kb = the molal boiling point constant (p. 432) m = the molality of the solution i = the number of ions present (covalent = 1, ionic = total number of ions) Example Problem 1: Calculate the boiling point elevation of adding 150 g of NaCl into 250 g of water. m = 150g/ 58.44 g/mol / .250 kg= 10.27 m Dtb = Kb X m X i = (.51oC/m) (10.27 m) (2) = 10.48 oC = Since the normal boiling point of water is 100oC, we add the 10.48 oC and get: = 110.48oC Example Problem 2: Calculate the boiling point elevation of adding 100 g of CH3OH into 500 g of the non- electrolyte, camphor. m = 100g/ 32.05 g/mol / .500 kg = 6.24 m Dtb = Kb X m X i = (5.61oC/m) (6.24 m) (1) = 35.01oC = Since the normal boiling point of camphor is 207.4oC, we add the 35.01oC : = 242.41 oC 4) Osmotic Pressure - The external pressure that must be applied to stop osmosis. o Osmosis- The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration. (p.436) o Semipermeable Membrane- Allows the passage of some particles while blocking the passage of others. Questions for the day: 1) Why does a bottle of nail polish remover evaporate more quickly than a glass of water? Vapor Pressure Lowering. There is less force holding the liquid nail polish remover to stay as a liquid than water. Water is a nonvolatile substance, where nail polish remover is a volatile substance. 2) Why do street crews add CaCl2 to water before a snow-storm as opposed to an equal concentration of NaCl? Freezing Point Depression. +2 -1 -1 +1 -1 i = 3 for CaCl2 àCa + Cl + Cl i = 2 for NaCl à Na + Cl o o Dtf = Kf * m * i = (-1.86 C/m) (m) (3) Dtf = Kf * m * i = (-1.86 C/m) (m) (2) *Since the Kf and m are the same, the number of ionis (i) makes the biggest difference in the freezing point temperature. 3) Why do some people add salt to their boiling pot of spaghetti noodles? Boiling Point Elevation. This allows for the boiling point to just slightly increase enough to stop the bubbles form bubbling over the pot of noodles. 4) Why do IV’s in hospitals have a saline solution (salt water) instead of distilled water? What would happen if the solution would become more concentrated? What would happen if it was pure (distilled) water? Osmotic Pressure. The concentration of blood is also the same concentration. If the solution becomes more concentrated, the cells can shrivel up and die. If only distilled water is used, the cells could expand and burst. .
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