1. Colligative Properties
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2. Colligative Properties
Collective effect of the number of solute particles
Not on the nature of the solute
Four common types
Boiling Point Elevation
Freezing point Depression
Vapor Pressure Lowering (Raoult’s Law)
Osmotic Pressure
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3. Colligative Properties (continued)
Properties of a solvent that depend on total concentration of the solute
Van’t Hoff factor i is a measure of the degree of dissociation of the solute in a solvent
Determined experimentally
Mathematically, given n particles and the fraction α that dissociates
Solvent
Normal BP (°C)
Kb (°C/m)
Normal FP (°C)
Kf (°C/m)
Water, H2O
100
0.52
1.86
Benzene, C6H6
80.1
2.53
5.5
5.12
Ethanol, C2H5OH
78.4
1.22
-114.6
1.99
Carbon Tetrachloride CCl4
76.8
5.02
-22.3
29.8
Chloroform, CHCl3
61.2
3.63
-63.5
4.68
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4. Boiling Point Elevation
Boiling point increases proportionate to the number of solute particles per mole of solvent particles
Normal boiling point is the temperature at which vapor pressure is at 1atm
With more solute, temperature must be increased to induce boiling
Note: The vapor pressure curve of a dilute solution lies below that of the pure solvent therefore the P is the decrease of vapor pressure at Tb (boiling point).
Tb is the change in temperature necessary to hold the vapor pressure at 1 atm (Tb is increase in boiling point caused by addition of solute to pure solvant)
Mathematical derivation!!!
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5. Boiling Point Elevation
At Low Concentrations
For very dilute solutions:
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6. Example
When 5.5 g of biphenyl (C12H10) is dissolved in 100g of benzene, the boiling point increases by 0.903ºC. Calculate Kb for benzene. (Biphenyl M.M. = g/mol)
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7. Example
When 0.494g of K3Fe(CN)6 is dissolved in g of H2O, the freezing point is found to be oC. How many ions are present for each formula unit of K3Fe(CN)6 dissolved?
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8. Workshop on Boiling Point Elevation
1. When a 11.2 G sample of sulfur was dissolved in 40.0 gof CS2, the boiling point elevation of CS2 is 2.63ºC. What is the molecular weight of sulfur in the solution? What is the formula of molecular sulfur?
Lanthanum (III) chloride, LaCl3, like many soluble salts, completely dissociates into ions in dilute aqueous solutions.
Suppose g of LaCl3 will dissolve in g of H2O, what will be the boiling point of the solution at 1 atm?
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9. Freezing Point Depression
Freezing point declines relative to molality of the solute
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10. Vapor Pressure Lowering
Similar to mole fraction calc from gas laws
Actual vapor pressure of solvent is only fraction of what it would be if a pure liquid
This is because of attractions between solvent and solute
Leads to Raoult’s Law
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11. Raoult’s Law Works with ideal solutions Conditions
Vapor pressure must be nonzero
Solute nonvolatile
Temperature constant
Shows that solute attracts solvent molecules, reducing number of solvents that escape into the vapor phase
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12. Osmotic Pressure
Net movement of solvent molecules from less concentrated solution to more concentrated solution
Pressure required to prevent osmosis is known as osmotic pressure
Exact equation (based on chemical potential in solution) yields:
If we assume an ideal solution at low concentration, we can further approximate:
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13. Osmosis is the phenomenon of solvent flow through a
semipermeable membrane to equalize the solute concentration on both sides of the membrane.
Osmotic pressure is a colligative property of a solution equal to the pressure that, when applied to the solution just stops the flow of solute.
1. Solvent flows in and out of the membrane but the solute does not.
2. The volume of the solution inside the membrane increases, stretching the membrane, until equilibrium is reached.
3. The pressure on the solution side of the membrane is greater than atmospheric pressure on the surface of the pure solvent.
4. The different between these two pressures is osmotic
pressure.
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14. Lecture Questions:
1. At 25ºC, the vapor pressure of C6H6 is atm. When g of an unknown volatile substance is dissolved in g of benzene, the vapor pressure of the solution, at 25ºC, is atm. Calculate the molar mass of the solute.
What is the osmotic pressure at 25º C of an isotonic saline solution that contains g NaCl in 100 mL of aqueous solution? Assume i is ideal.
3. At 25ºC, the freezing point of a NaCl aqueous solution is ºC. Calculate the osmotic pressure this solution has on a semi-permeable membrane if the concentration of the solution is equivalent to the molality.
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15. Workshop on Colligative Properties
1. Determine the vapor pressure of a solution of 92.1 g of glycerin, C3H5(OH)3, in g of ethanol at 40 C. The vapor pressure of pure ethanol is atm at 40 C, and glycerin is essentially nonvolatile.
2. Find the boiling point of a solution of 92.1 g of iodine in g of chloroform.
3. Calculate the freezing point of a solution of g of calcium chloride in 175 g of water, assuming complete dissociation by the solute.
4. Determine the osmotic pressure of a solution with a volume of L that contains 5.0 g of methanol in water at 37 C.
5. List the following aqueous solutions in order of their expected freezing points: m CaCl2, 0.15 m NaCl, 0.10 m HCl, m HC2H3O2, and 0.10 m C12H22O11.
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16. Workshop continued:
6. A solution of 4.00 g of a nonelectrolyte dissolved in 55.0 g of benzene is found to freeze at 2.32 C. What is the molar mass of this compound?
L of an aqueous solution that contains 10.0 g of hemoglobin has an osmotic pressure of 5.9 torr at 22 C. What is the molar mass of hemoglobin?
8. A solution of 35.7 g of a nonelectrolyte in g of chloroform has a boiling point of 64.5 C. What is the molar mass of this compound?
9. An organic compound has a composition of 93.46% C and 6.54% H by mass. A solution of g of this compound in 1.10 g of camphor melts at C. The melting point of pure camphor is C, and its freezing point depression constant is 37.7 C m-1. What is the molecular formula of the solute?
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17. Colloids
Dispersion of particles of one substance throughout another substance or solution
Heterogeneous mixtures
Tyndall effect
Scattering of light by colloidal-size particles
Particles in the range of 40 – 900nm
This is in (or near) the visible spectrum
Examples: Starch in water, fog, eye layers
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18. Types of COLLOIDS Continous Dispersed Name Example Phase Phase
Gas Liquid Aerosol Fog, mist
Gas Solid Aerosol Smoke
Liquid Gas Foam Whipped Cream
Liquid Liquid Emulsion Mayonnaise
(oil dispersed
in water
Liquid Solid Sol AgCl(s) dispersed
in H2O
Solid Gas Foam Pumice, plastic
foams
Solid Liquid Gel Jelly, Opal
(mineral with
liquid inclusions)
Solid Solid Solid sol Ruby glass
(glass with
dispersed metal)
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19. Types of Colloids Hydrophilic Colloid Hydrophobic Colloid Coagulation
Strong attraction between the dispersed phase and continuous phase
Hydrophobic Colloid
Lack of attraction between the dispersed phase and continuous phase
Coagulation
Process by which dispersed phase is made to aggregate
Separates from the continuous phase
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20. Association Colloid Micelle Sodium Lauryl Sulfate: CH3(CH2)11OSO3- Na+
Colloid-size particle favored in water by association of molecules or ions
Each particle has a hydrophobic and hydrophilic end
Sodium Lauryl Sulfate: CH3(CH2)11OSO3- Na+
Sterate ion: CH3(CH2)16COO- (shown as stearic acid)
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21. The End End of Chem 101 Final Wednesday May 29 2013 150 points
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