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.
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