Prepared by PhD Halina FalfushynskaPrepared by PhD Halina Falfushynska

Lecture 8. Colligative Lecture 8. Colligative properties of solutionsproperties of solutions

A space-filling model of the water molecule.

GENERAL PROPERTIES OF SOLUTIONSGENERAL PROPERTIES OF SOLUTIONS1. A solution is a homogeneous mixture of two or

more components.

2. It has variable composition.

3. The dissolved solute is molecular or ionic in size.

4. A solution may be either colored or colorless nut is generally transparent.

5. The solute remains uniformly distributed throughout the solution and will not settle out through time.

6. The solute can be separated from the solvent by physical methods.

Polar water molecules interact with the positive and negative ions of a salt,

assisting with the dissolving process.

Electrical Conductivity of Ionic Solutions

Electrical Conductivity

Comparison of a Concentrated and Dilute Solution

Comparison of an Unsaturated and Saturated Solution

Molarity (Concentration of Solutions)= M

M = = Moles of Solute MolesLiters of Solution L

solute = material dissolved into the solvent

In air (gas), Nitrogen is the solvent and oxygen, carbon dioxide, etc. are the solutes.

In sea water (liquid), Water is the solvent, and salt, magnesium chloride, etc. are the solutes.

In brass , Copper is the solvent (90%), and Zinc is the solute(10%)

MOLALITYMOLALITY• Molality = moles of solute per kg of solvent

• m = nsolute / kg solvent

• If the concentration of a solution is given in terms of molality, it is referred to as a molal solutionmolal solution.

Q. Calculate the molality of a solution consisting of Q. Calculate the molality of a solution consisting of 25 g of KCl in 250.0 mL of pure water at 2025 g of KCl in 250.0 mL of pure water at 20ooC?C?

First calculate the mass in kilograms of solvent using the density of solvent:

250.0 mL of H2O (1 g/ 1 mL) = 250.0 g of H2O (1 kg / 1000 g) = 0.2500 kg of H2O

Next calculate the moles of solute using the molar mass:

25 g KCl (1 mol / 54.5 g) = 0.46 moles of solute

Lastly calculate the molality:

m = n / kg = 0.46 mol / 0.2500 kg = 1.8 1.8 mm (molal) solution (molal) solution

HEAT EFFECT ON THE GAS DILUTION IN WATER

PRESSURE AFFECTS GAS SOLUBILITY

HENRY’S LAW

m = kP

m – mass of soluble gas;

k – Henry’s constant;

P – partial gas pressure.

СО2 pressure in bottle is 4 atm.

Decreases of pressure of saturated vapor Decreases of pressure of saturated vapor under solutionunder solution

Colligative: particles are particles

• Colligative comes from colligate – to tie together

• Colligative properties depend on amount of solute but do not depend on its chemical identity

• Solute particles exert their effect merely by being rather than doing

• The effect is the same for all solutes

COLLIGATIVE PROPERTIES FOR NONVOLATILE SOLUTES:

• Vapour pressure is always lower

• Boiling point is always higher

• Freezing point is always lower

• Osmotic pressure drives solvent from lower concentration to higher concentration

NON-VOLATILE SOLUTES AND RAOULT’S LAW

• Vapor pressure of solvent in solution containing non-volatile solute is always lower than vapor pressure of pure solvent at same T

–At equilibrium rate of vaporization = rate of condensation–Solute particles occupy volume reducing rate of evaporation the

number of solvent molecules at the surface–The rate of evaporation decreases and so the vapor pressure above

the solution must decrease to recover the equilibrium

Molecular view of Raoult’s law:Boiling point elevation

• In solution vapor pressure is reduced compared to pure solvent

• Liquid boils when vapor pressure = atmospheric pressure

• Must increase T to make vapor pressure = atmospheric

Colligative Properties – BP Elevation• The addition of a nonvolatile solute

causes solutions to have higher boiling points than the pure solvent.

– Vapor pressure decreases with addition of non-volatile solute.

Higher temperature is needed in order for vapor pressure to equal 1 atm.

MOLECULAR VIEW OF RAOULT’S LAW:FREEZING POINT DEPRESSION

– Ice turns into liquid– Lower temperature to regain balance– Depression of freezing point

• Depends on the solute only being in the liquid phase– Fewer water

molecules at surface: rate of freezing drops

Colligative Properties - Freezing Pt Depression

Freezing point of the solution is lower than that of the pure solvent.

• The addition of a nonvolatile solute causes solutions to have lower freezing points than the pure solvent.

• Solid-liquid equilibrium line rises ~ vertically from the triple point, which is lower than that of pure solvent.

RAOULT’S LAW

• Vapor pressure above solution is vapor pressure of solvent times mole fraction of solvent in solution

• Vapour pressure lowering follows:

solvsolvso XPP ln

solutesolvso XPP ln

MAGNITUDE OF ELEVATION

• Depends on the number of particles present

• Concentration is measured in molality (independent of T)

• Kb is the molal boiling point elevation constant

mKT bb

Boiling point elevation (ebullioscopy)

• The boiling point of a pure solvent is increased by the addition of a non-volatile solute, and the elevation can be measured by ebullioscopy.

• Here i is the van't Hoff factor as above, Kb is the ebullioscopic constant of the solvent (equal to 0.512°C kg/mol for water), and m is themolality of the solution

MAGNITUDE OF DEPRESSION

• Analagous to boiling point, the freezing point depression is proportional to the molal concentration of solute particles

• For solutes which are not completely dissociated, the van’t Hoff factor is applied to modify m:

mKT ff

imKT ff

Freezing point depression (cryoscopy)

• The freezing point of a pure solvent is lowered by the addition of a solute which is insoluble in the solid solvent, and the measurement of this difference is called cryoscopy.

• Here Kf is the cryoscopic constant, equal to 1.86°C kg/mol for the freezing point of water. Again i is the van't Hoff factor and m the molality.

OSMOSIS: MOLECULAR DISCRIMINATION

• A semi-permeable membrane discriminates on the basis of molecular type– Solvent molecules pass through– Large molecules or ions are blocked

• Solvent molecules will pass from a place of lower solute concentration to higher concentration to achieve equilibrium

OSMOTIC PRESSURE

• Solvent passes into more conc solution increasing its volume

• The passage of the solvent can be prevented by application of a pressure

• The pressure to prevent transport is the osmotic pressure

CALCULATING OSMOTIC PRESSURE

• The ideal gas law states

• But n/V = M and so

• Where M is the molar concentration of particles and Π is the osmotic pressure

nRTPV MRT

Determining molar mass

• A solution contains 20.0 mg insulin in 5.00 ml develops an osmotic pressure of 12.5 mm Hg at 300 K

RTM

MK

KmolatmL

mmHgmmHgM 41068.6

3000821.0

76015.12

VOLATILE SOLUTE: TWO LIQUIDS

• Total pressure is the sum of the pressures of the two components

BAtotal PPP

BBAAtotal XPXPP

OSMOMETERp = p = ghgh

Colligative Properties - Osmosis• Osmosis plays an important role

in living systems:– Membranes of red blood cells are

semipermeable.

• Placing a red blood cell in a hypertonic solution (solute concentration outside the cell is greater than inside the cell) causes water to flow out of the cell in a process called CRENATION.

Colligative Properties

• Placing a red blood cell in a hypotonic solution (solute concentration outside the cell is less than that inside the cell) causes water to flow into the cell.– The cell ruptures in a process called

HEMOLYSIS.