COLLIGATIVE PROPERTIES. Engr. Yvonne Ligaya F. Musico 1

Transcription

1 COLLIGATIVE PROPERTIES Engr. Yvonne Ligaya F. Musico 1

2 Colligative Properties Properties that depend on the collective effect of the number of solute particles. Engr. Yvonne Ligaya F. Musico 2

3 COLLEGATIVE PROPERTIES 1. LOWERING THE VAPOR PRESSURE (VAPOR PRESSURE REDUCTION) 2. BOILING POINT ELEVATION 3. FREEZING-POINT DEPRESSION 4. OSMOTIC PRESSURE Engr. Yvonne Ligaya F. Musico 3

4 1. LOWERING THE VAPOR PRESSURE (VAPOR PRESSURE REDUCTION) Vapor Pressure the pressure exerted by the vapor. Non-volatile a substance that has no measurable vapor pressure. Volatile one that exhibits a vapor pressure. Engr. Yvonne Ligaya F. Musico 4

5 RAOULT S LAW It states that the partial pressure exerted by solvent vapor above a solution equals the product of the mole fraction of the solvent in the solution times the vapor pressure of the solvent. Ideal Solution - A solution that obeys Raoult s law Engr. Yvonne Ligaya F. Musico 5

6 RAOULT S LAW Where: P A = X A P A o P A = partial pressure exerted by the solvent above the solution. X A = mole fraction of the solvent in the solution P A o = vapor pressure of the pure solvent Engr. Yvonne Ligaya F. Musico 6

7 Note: In a solution that has a non volatiles solute, the vapor pressure of the solution is due only to the solvent. We can use P A to refer to either the solvent or the solution. Engr. Yvonne Ligaya F. Musico 7

8 Sample Problem 1 The vapor pressure of water is 17.5 torr at 20 o C. Imagine holding the temperature constant while adding X H 2O = 0.80 and X C 6H12O6 = Find the partial vapor pressure of water over the solution. Engr. Yvonne Ligaya F. Musico 8

9 Solution P H 2 O X H 2 O p H 2 O O (.80)(17.5 torr) 14 torr Engr. Yvonne Ligaya F. Musico 9

10 Sample Problem 2 What is the vapor pressure of a 20% by weight glucose (C 6 H 12 O 6 ) solution in water at 29 o C if the vapor pressure of water is (P o ) is torr? Engr. Yvonne Ligaya F. Musico 10

11 Solution Basis: 100 g of solution m glucose = 20 grams m water = 80 grams 80g nh ( ) o solvent mol 18g / mol n solution n C 6 n H C 6 12 H O 6 12 ( solute) O 6 ( solute) 20g 0.11 mol 180g / mol n 4.55 H 2 o( solvent) Engr. Yvonne Ligaya F. Musico 11

12 Solution X H 2 o mol mol P (30 torr) P torr Engr. Yvonne Ligaya F. Musico 12

13 Practice Exercise Glycerine, C 3 H 8 O 3, is a non-volatile non-electrolyte with a density of 1.26 g/ml at 25 o C. Calculate the vapor pressure at 25 o C of the solution made by adding 50.0 ml of glycerine to 500 ml of water. The vapor pressure of pure water at 25 o C is 23.8 torr (Appendix B, Chemistry: The Central Science by Brown, Lemay, Bursted) Answer : 23.2 torr Engr. Yvonne Ligaya F. Musico 13

14 2. BOILING POINT ELEVATION The increase in boiling point relative to that of the pure solvent, T b, is directly proportional to the number of solute particles per mole of solvent molecules. Engr. Yvonne Ligaya F. Musico 14

15 2. BOILING POINT ELEVATION We know that molality expresses the number of moles of solute per 1000 g of solvent, which represents a fixed number of moles of solvent. Thus: T b = k b m Where: T b = increase in boiling point, o C k b = molality boiling-point elevation constant o C/m m = molality, m Engr. Yvonne Ligaya F. Musico 15

16 2. BOILING POINT ELEVATION The boiling-point elevation is proportional to the number of particles present in a given quantity of solution. Engr. Yvonne Ligaya F. Musico 16

17 3. FREEZING POINT DEPRESSION Freezing Point the temperature at which the first crystals of pure solvent begin to form in equilibrium with the solution. The lower the vapor pressure of the solution relative to the liquid also affects the freezing point of the solution Engr. Yvonne Ligaya F. Musico 17

18 3. FREEZING POINT DEPRESSION The decrease in freezing point, T f, is directly proportional to the molality of the solute. Where: T f = k f m T f = decrease in freezing point, o C k f = molal freezing-point-depression constant, o C/m m = molality, m Engr. Yvonne Ligaya F. Musico 18

19 Table 1. Molal Boiling-point-elevation and Freezing-pointdepression Constants Solvent Normal Boiling Point ( o C) k b ( o C/m) Normal Freezing Point ( o C) k f ( o C/m) Water, H 2 O Benzene, C 6 H Ethanol, C 2 H 5 OH Carbon tetrachloride, CC l Chloroform, CHCl Engr. Yvonne Ligaya F. Musico 19

20 Sample Problem 1 Calculate the freezing point and boiling point of a solution which contains 35 g sucrose (C 12 H 22 O 11 ) in 370 g H 2 O. Engr. Yvonne Ligaya F. Musico 20

21 Solution 35 g moles solute 342 g / mol m kg solvent 0.37 kg m k b = o C/m (from table 1, for H 2 O) Engr. Yvonne Ligaya F. Musico 21

22 Solution T b k b m (.512 o C / m)(0.277 m) o C T b T b o o Tb 100 C C i o C Engr. Yvonne Ligaya F. Musico 22

23 Solution k f = 1.86 o C/m (from table 1, for H 2 O) T f k f m (1.86 o C / m)(0.277 m) o C T f T f o o T f 0 C C i o C Engr. Yvonne Ligaya F. Musico 23

24 Problem Sample 2 What is the boiling point and freezing point of a solution prepared by dissolving 2.40 g of biphenyl (C 12 H 10 ) in 75.0 g of benzene. The molecular weight biphenyl is 154 g/mole. Engr. Yvonne Ligaya F. Musico 24

25 Practice Exercise A solution prepared by dissolving g of an unknown non-volatile solution 30.0 g of Carbon tetrachloride has a boiling point that is o C higher than that of pure CCl 4. What is the molecular weight of the solute? Engr. Yvonne Ligaya F. Musico 25

26 4. OSMOTIC PRESSURE Definition of Terms Osmosis the passage of solvent through a semipermeable membrane from a solution of lesser concentration to one of higher concentration. Osmotic Pressure the pressure that develops in the process in which there is net movement of solvent molecules through a semipermeable membrane that separates two solutions, the movement occurring to the direction of the more concentrated solution. Engr. Yvonne Ligaya F. Musico 26

27 Tonicity Isotonic two solutions have equal osmotic pressure Hypotonic one solution is of lower osmotic pressure with respect to the more concentrated solution. Hypertonic the more concentrated solution with respect to the dilute solution. Engr. Yvonne Ligaya F. Musico 27

28 Van t Hoff Equation ΠV = nrt Where: Π = osmotic pressure, atm n = the number of moles of solute, mol V = volume, L R = gas constant, L.atm/mol.K) T = absolute temperature, K Engr. Yvonne Ligaya F. Musico 28

29 Osmotic pressure n V RT MRT Engr. Yvonne Ligaya F. Musico 29

30 Sample Problem 1 Find the osmotic pressure of blood at normal body temperature (37 o C). If the blood behaves as if it was a M solution of a non ionizing solute. Engr. Yvonne Ligaya F. Musico 30

31 Solution mol L atm MRT L mol K 310 K 7. atm Engr. Yvonne Ligaya F. Musico 31

32 Sample Problem 2 An aqueous solution contains 30 g of a protein in 1.0 L. The osmotic pressure of the solution is atm at 25 o C. What is the molecular weight of protein? Engr. Yvonne Ligaya F. Musico 32

33 Exercises 1. When 10.6 g of a non volatile substance is dissolved in 740 g of ether, its boiling point is raised by o C. What is the molecular weight of the substance? The molal boiling point constant for ether is 2.11 o C-kg/mol 2. A solution was made up by dissolving 3.75 g of a pure hydrocarbon in 95 g of acetone. The boiling point of pure acetone was observed to be o C and of the solution If the molal boiling point constant of acetone is 1.71 o C-kg/mol, what is the approximate molecular weight of the hydrocarbon? 3. The freezing point of a sample napthalene was found to be 80.6 o C. When g of a substance is dissolved in 7.03 g napthalene, the solution has a freezing point of 75.2 o C. What is the molecular weight of the solute? The molal freezing point of napthalene is 0.80 o C-kg/mol Engr. Yvonne Ligaya F. Musico 33

34 Exercises 4. The vapor pressure of pure H 2 O at 26 o C is torr. What is the vapor pressure of solution which contains 20 g of glucose (C 6 H 12 O 6 ) in 70 g H 2 O? 5. Calculate the osmotic pressure of an aqueous solution which contains 4.0 g of glucose (C 6 H 12 O 6 ) in 250 ml of solution at 25 o C. 6. At 30 o C pure benzene (MW = 78.1 g/mol) has a vapor pressure of torr. Dissolving 15 g of a non volatile solute in 250 g of benzene produced a solution having a vapor pressure of torr. Determine the approximate molecular weight of the solute. Engr. Yvonne Ligaya F. Musico 34

35 COLLOIDS Colloids or Colloidal dispersion intermediate type of dispersion or suspensions. the dividing line between solutions and heterogeneous mixture. Tyndall Effect light scattering by colloidal dispersion. Hydrophobic water-fearing colloids Hydrophilic water-loving colloids Adsorption means to adhere to the surface Absorption which means to pass through interior Engr. Yvonne Ligaya F. Musico 35

36 Thanks for listening Engr. Yvonne Ligaya F. Musico 36