Prentice Hall © 2003Chapter 13 Mass Percentage, ppm, and ppb Definitions: Ways of Expressing Concentration

Prentice Hall © 2003Chapter 13 Example: How would you prepare 425 g of an aqueous solution containing 2.40% by mass of sodium acetate, NaC 2 H 3 O 3 ? Ans: Mass of NaC 2 H 3 O 3 = 10.2 g Mass of H 2 O = mass of solution - mass of NaC 2 H 3 O 3 = 415 g

Prentice Hall © 2003Chapter 13 Exercise: Concentrated aqueous nitric acid has 69.0% by mass of HNO 3 and has a density of 1.41 gcm -3. What volume of this solution contains 14.2 g of HNO 3 ?

Prentice Hall © 2003Chapter 13 Also mgl -1

Prentice Hall © 2003Chapter 13 Also μgl -1

Prentice Hall © 2003Chapter 13 Exercise: Seawater contains g of dissolved oxygen, O 2, per litre. The density of seawater is 1.03 gcm -3. What is the concentration of oxygen, in ppm? Ans: 6.2 ppm

Prentice Hall © 2003Chapter 13 Mole Fraction, Molarity, and Molality

Prentice Hall © 2003Chapter 13 Converting between molarity (M) and molality (m) requires density. Exercise: 0.2 mol of ethylene glycol is dissolved in 2000 g of water. Calculate the molality

Prentice Hall © 2003Chapter 13 Example: What is the molality of a solution containing 5.67 g of glucose, C 6 H 12 O 6 (M r = g), dissolved in 25.2 g of water? (Calc. the mole fractions of the components as well). Solution:  Think about the solute! glucose (express in moles)  Think about the solvent! water (express in kilograms) Ans: 1.25 m

Prentice Hall © 2003Chapter 13 An aqueous solution is M Pb(NO 3 ) 2. What is the molality of lead nitrate, Pb(NO 3 ) 2, in this solution? The density of the solution is g/mL. (Molar mass of Pb(NO 3 ) 2 = g) Solution:  Mass of solution = density x volume  Calculate mass of Pb(NO 3 ) 2, ie, moles x Mr  Mass of H 2 O = mass of solution – mass of Pb(NO 3 ) 2  Molality = m Pb(NO 3 ) 2 Example: Converting molarity to molality

Prentice Hall © 2003Chapter 13 Colligative properties - depend only on the number of particles in solution and not on their identity. So NaCl(aq)  a + (aq) + Cl - (aq) K 2 SO 4 (aq)  2K + (aq) + SO 4 2- (aq) C 12 H 22 O 11 (aq)  C 12 H 22 O 11 (aq) Colligative Properties

Prentice Hall © 2003Chapter 13 Examining the effect of adding a non-volatile solute to a solvent on: 1. vapor pressure 2. boiling point 3. freezing point 4. osmosis

Prentice Hall © 2003Chapter 13  Examples are: anti-freeze in the radiator water in a car prevents freezing in winter and boiling in summer; snow is melted by adding salt on sidewalks and streets

Prentice Hall © 2003Chapter 13 Lowering Vapor Pressure VP lowering depends on the amount of solute.

Prentice Hall © 2003Chapter 13

Prentice Hall © 2003Chapter 13 Lowering Vapor Pressure Raoult’s law: P soln = X solvent P o solvent Recall Dalton’s Law: P total = P A + P B + P C +….P N

Prentice Hall © 2003Chapter 13 Ideal solution - obeys Raoult’s law Raoult’s law is to solutions what the ideal gas law is to gases Raoult’s law breaks down when the solvent-solvent and solute- solute intermolecular forces are greater than solute-solvent intermolecular forces For liquid-liquid solutions where both components are volatile, a modified form of Raoult’s law applies: P total = P A + P B = X A P o A + X B P o B

Prentice Hall © 2003Chapter 13 Example: Predict the vapour pressure of a solution prepared by mixing 35 g solid Na 2 SO 4 (Mr = 142 g/mol) with 175 g water at 25 o C. The vapour pressure of pure water at 25 o C is torr. Ans: 22.1 torr

Prentice Hall © 2003Chapter 13 Exercise: The hydrocarbon limonene is the major constituent of lemon oil. A solution of limonene in 78.0 g of benzene had a vapour pressure of 90.6 mm Hg at 25 o C, and the vapour pressure of pure benzene at 25 o C is 95.2 mm Hg. What is its mass and molecular formula? Ans: C 10 H 16

Prentice Hall © 2003Chapter 13  As with gases, ideal behaviour for solutions is never perfectly achieved  Nearly ideal behaviour is observed if solute-solute, solvent-solvent and solute- solvent interactions are very similar

Prentice Hall © 2003Chapter 13 Boiling-Point Elevation Goal: interpret the phase diagram for a solution.  Non-volatile solute lowers the vapor pressure  Therefore the triple point - critical point curve is lowered.

Prentice Hall © 2003Chapter 13 Molal boiling-point-elevation constant, K b, expresses how much  T b changes with molality, m:

Prentice Hall © 2003Chapter 13 Freezing Point Depression

Prentice Hall © 2003Chapter 13 Freezing Point Depression Colligative Properties

Prentice Hall © 2003Chapter 13 Example: How many grams of ethanol, C 2 H 5 OH, must be added to 37.8 g of water to give a freezing point of o C? Solution: Water is the solvent and ethanol the solute From table 13.4,  Tf = 0.15 o C; K f for water is 1.86 o C/m

Prentice Hall © 2003Chapter 13 Colligative properties of ionic solutions  T f = iK f m where i is the no. of ions resulting from each formula unit

Prentice Hall © 2003Chapter 13 Example: Estimate the freezing point of a m aqueous solution of aluminium sulphate, Al 2 (SO 4 ) 3. Assume the value of i based on the formula of the compound. Ans: o C

Prentice Hall © 2003Chapter 13 Osmosis Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane. Osmosis: the movement of a solvent from low solute concentration to high solute concentration.

Prentice Hall © 2003Chapter 13 Osmosis Eventually the pressure difference between the arms stops osmosis.

Prentice Hall © 2003Chapter 13 Osmosis Osmotic pressure, , is the pressure required to stop osmosis: Isotonic solutions are solutions….?

Prentice Hall © 2003Chapter 13 Hypotonic solutions are solutions….? Hypertonic solutions are solutions…? Osmosis is spontaneous. Red blood cells are surrounded by semipermeable membranes.

Prentice Hall © 2003Chapter 13 Example: The formula for low-molecular weight starch is (C 6 H 10 O 5 ) n, where n averages 2x10 2. When g of starch is dissolved in 100 mL of water solution, what is the osmotic pressure at 25 o C?  = atm

Prentice Hall © 2003Chapter 13 Exercise: Fish blood has an osmotic pressure equal to that of seawater. If seawater freezes at -2.3 o C, what is the osmotic pressure of the blood at 25 o C? Ans: 30 atm

Prentice Hall © 2003Chapter 13 Crenation: –red blood cells placed in hypertonic solution (relative to intracellular solution); –The cell shrivels or swells up?

Prentice Hall © 2003Chapter 13 Hemolysis: there is a higher solute concentration in the cell; What happens to the cell?

Prentice Hall © 2003Chapter 13 Osmosis

Prentice Hall © 2003Chapter 13 Hypertonic solution Hypotonic solution

Prentice Hall © 2003Chapter 13 To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic.

Prentice Hall © 2003Chapter 13 –Cucumber placed in NaCl solution loses water to shrivel up and become a pickle. –Limp carrot placed in water becomes firm because water enters via osmosis. –Salty food causes retention of water and swelling of tissues (edema). –Water moves into plants through osmosis. –Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die).

Prentice Hall © 2003Chapter 13 Active transport is the movement of nutrients and waste material through a biological system. Active transport is not spontaneous.

Prentice Hall © 2003Chapter 13 Colloids A colloid is a dispersion of particles of one substance (the dispersed phase) throughout another substance or solution (the continuous phase) Examples….??

Prentice Hall © 2003Chapter 13 The particle sizes range from ~1 x 10 3 pm to 2 x 10 5 pm in size

Prentice Hall © 2003Chapter 13 Although a colloid appears to be homogeneous because the dispersed particles are quite small, it can be distinguished from a true solution by its ability to scatter light This is called the……effect?

Prentice Hall © 2003Chapter 13 Left: vessel containing colloid; Right: true solution

Prentice Hall © 2003Chapter 13 Aerosols – liquid droplets or solid particles dispersed in a gas e.g. fog and smoke Emulsion – liquid droplets dispersed throughout another liquid e.g. butterfat in milk Sol – solid particles dispersed in a liquid e.g. AgCl(s) in H 2 O

Prentice Hall © 2003Chapter 13 Colloids in which the continuous phase in water can be hydrophilic (e.g. protein molecules) or hydrophobic colloids (Au particles in water).

Prentice Hall © 2003Chapter 13 How does soap stabilise oil in water? And how do we digest fats in our digestive systems?

Prentice Hall © 2003Chapter 13 Removal of colloidal particles Because of their small size, colloidal particles tend to be difficult to remove by processes such as filtration. Thus enlargement of particles by coagulation is required Heating or adding an electrolyte can cause coagulation

Prentice Hall © 2003Chapter 13 Heating increases collisions and hence particle size Electrolytes neutralise surface charges and reduce repulsions e.g. Alum in water purification, clay deposits in deltas Semi-permeable membranes can also be used to separate ions from colloidal particles (dialysis e.g. waste removal from blood by kidneys)