5/25/2012 Colligative Properties of Solutions . Colligative Properties: • Solution properties that depend on concentration of solute particles, not the identity of particles. Previous example: vapor pressure lowering. Consequences: change in b.p. and f.p. of solution. © 2012 by W. W. Norton & Company Solute Concentration: Molality . Changes in boiling point/freezing point of solutions depends on molality: moles of solute m kg of solvent • Preferred concentration unit for properties involving temperature changes because it is independent of temperature. © 2012 by W. W. Norton & Company 1 5/25/2012 Calculating Molality Starting with: a) Mass of solute and solvent. b) Mass of solute/ volume of solvent. c) Volume of solution. © 2012 by W. W. Norton & Company Sample Exercise 11.8 How many grams of Na2SO4 should be added to 275 mL of water to prepare a 0.750 m solution of Na2SO4? Assume the density of water is 1.000 g/mL. © 2012 by W. W. Norton & Company 2 5/25/2012 Boiling-Point Elevation and Freezing-Point Depression . Boiling Point Elevation (ΔTb): • ΔTb = Kb∙m • Kb = boiling point elevation constant of solvent; m = molality. Freezing Point Depression (ΔTf): • ΔTf = Kf∙m • Kf = freezing-point depression constant; m = molality. © 2012 by W. W. Norton & Company Sample Exercise 11.9 What is the boiling point of seawater if the concentration of ions in seawater is 1.15 m? © 2012 by W. W. Norton & Company 3 5/25/2012 Sample Exercise 11.10 What is the freezing point of radiator fluid prepared by mixing 1.00 L of ethylene glycol (HOCH2CH2OH, density 1.114 g/mL) with 1.00 L of water (density 1.000 g/mL)? The freezing-point-depression constant of water, Kf, is 1.86°C/m. © 2012 by W. W. Norton & Company The van’t Hoff Factor . Solutions of Electrolytes: • Need to correct for number of particles formed when ionic substance dissolves. van’t Hoff Factor (i): • number of ions in formula unit. • e.g., NaCl, i = 2 . ΔTb = i∙Kb∙m & ΔTf = i∙Kf∙m . Deviations from theoretical value due to ion pair formation. © 2012 by W. W. Norton & Company 4 5/25/2012 Values of van’t Hoff Factors © 2012 by W. W. Norton & Company Sample Exercise 11.11 The salt lithium perchlorate (LiClO4) is one of the most water-soluble salts known. At what temperature does a 0.130 m solution of LiClO4 freeze? The Kf of water is 1.86°C/m; assume i = 2 for LiClO4 and the freezing point of pure water is 0.00°C. © 2012 by W. W. Norton & Company 5 5/25/2012 Osmosis . Osmosis: • Movement of solvent through semi-permeable membrane from region of low solute concentration to region of higher solute concentration. Osmotic Pressure (π): • Pressure required to halt flow of solvent through membrane due to osmosis. • π = iMRT (M = molarity of solution) © 2012 by W. W. Norton & Company Osmosis at the Molecular Level Direction of solvent flow. Osmotic Pressure (π) © 2012 by W. W. Norton & Company 6 5/25/2012 Osmosis: Medical Application © 2012 by W. W. Norton & Company Reverse Osmosis . Use of high pressure to move solvent across membrane from region of high solute concentration to region of lower solute concentration. Application: desalination/water purification. © 2012 by W. W. Norton & Company 7 5/25/2012 Molar Mass from Colligative Properties Calculations: Sample Exercise 11.13 At the beginning of this section, we mentioned that the concentration of solutes in a red blood cell is about a third of that of seawater—more precisely, about 0.30 M. If red blood cells are bathed in pure water, they swell, as shown in Figure 11.22(c). Calculate the osmotic pressure at 25°C of red blood cells across the cell membrane from pure water. © 2012 by W. W. Norton & Company 8 5/25/2012 Sample Exercise 11.15 What is the reverse osmotic pressure required at 20°C to purify brackish well water containing 0.355 M dissolved particles if the product water is to contain no more than 87 mg of dissolved solids (as NaCl) per liter? © 2012 by W. W. Norton & Company Sample Exercise 11.16 Eicosene is a molecular compound and nonelectrolyte with the empirical formula CH2. The freezing point of a solution prepared by dissolving 100 mg of eicosene in 1.00 g of benzene was 1.75°C lower than the freezing point of pure benzene. What is the molar mass of eicosene? (Kf for benzene is 4.90°C/m.) © 2012 by W. W. Norton & Company 9 5/25/2012 Sample Exercise 11.17 A molecular compound that is a nonelectrolyte was isolated from a South African tree. A 47 mg sample was dissolved in water to make 2.50 mL of solution at 25°C, and the osmotic pressure of the solution was 0.489 atm. Calculate the molar mass of the compound. 10 .