Solute Concentration: Molality

Transcription

1 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. 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. 1

2 Calculating Molality Starting with: a) Mass of solute and solvent. b) Mass of solute/ volume of solvent. c) Volume of solution. Sample Exercise 11.8 How many grams of Na 2 SO 4 should be added to 275 ml of water to prepare a m solution of Na 2 SO 4? Assume the density of water is g/ml. 2

3 Boiling-Point Elevation and Freezing-Point Depression Boiling Point Elevation (ΔT b ): ΔT b = K b m K b = boiling point elevation constant of solvent; m = molality. Freezing Point Depression (ΔT f ): ΔT f = K f m K f = freezing-point depression constant; m = molality. Sample Exercise 11.9 What is the boiling point of seawater if the concentration of ions in seawater is 1.15 m? 3

4 Sample Exercise What is the freezing point of radiator fluid prepared by mixing 1.00 L of ethylene glycol (HOCH 2 CH 2 OH, density g/ml) with 1.00 L of water (density g/ml)? The freezing-point-depression constant of water, K f, is 1.86 C/m. 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 ΔT b = i K b m & ΔT f = i K f m Deviations from theoretical value due to ion pair formation. 4

5 Values of van t Hoff Factors Sample Exercise The salt lithium perchlorate (LiClO 4 ) is one of the most water-soluble salts known. At what temperature does a m solution of LiClO 4 freeze? The K f of water is 1.86 C/m; assume i = 2 for LiClO 4 and the freezing point of pure water is 0.00 C. 5

6 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) Osmosis at the Molecular Level Direction of solvent flow. Osmotic Pressure (π) 6

7 Osmosis: Medical Application 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. 7

8 Molar Mass from Colligative Properties Calculations: Sample Exercise 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. 8

9 Sample Exercise What is the reverse osmotic pressure required at 20 C to purify brackish well water containing M dissolved particles if the product water is to contain no more than 87 mg of dissolved solids (as NaCl) per liter? Sample Exercise Eicosene is a molecular compound and nonelectrolyte with the empirical formula CH 2. 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? (K f for benzene is 4.90 C/m.) 9

10 Sample Exercise 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 atm. Calculate the molar mass of the compound. 10