Transcript Colligative Properties LACC Chem101

Colligative Properties
LACC Chem101
Colligative Properties
 Collective effect of the number of solute particles
 Not on the nature of the solute
 Four common types
1. Boiling Point Elevation
Tb  kb M
2. Freezing point Depression
T f   k f M
3. Vapor Pressure Lowering (Raoult’s Law)
PA   A Pa 
4. Osmotic Pressure
nRT
  MRT 
V
LACC Chem 101
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Colligative Properties (continued)
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 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
i   n  1     1    n  1
Solvent
Normal BP (°C) Kb (°C/m) Normal FP (°C) Kf (°C/m)
Water, H2O
100
0.52
0
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
LACC Chem 101
Boiling Point Elevation
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Tb  iKb m
 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!!!
LACC Chem 101
Boiling Point Elevation
 At Low Concentrations

P1
 slope
Tb
P1 1P1 1  n1 
Tb 

 

S
S
S  n1  n2 
 For very dilute solutions: n1
1  n1  n2
Tb  

S  n1  n2  n1S
m1
Kb 
S
Tb  Kb m
LACC Chem 101
n2
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Example
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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. = 154.2 g/mol)
LACC Chem 101
Example
When 0.494g of K3Fe(CN)6 is dissolved in 100.0 g of H2O, the
freezing point is found to be -0.093 oC. How many ions are
present for each formula unit of K3Fe(CN)6 dissolved?
LACC Chem 101
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Workshop on Boiling Point Elevation
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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?
2. Lanthanum (III) chloride, LaCl3, like many soluble salts,
completely dissociates into ions in dilute aqueous solutions.
H 2O
LaCl3  s  
 La 3  aq   Cl   aq 
Suppose 0.2453 g of LaCl3 will dissolve in 10.00 g of H2O, what
will be the boiling point of the solution at 1 atm?
LACC Chem 101
Freezing Point Depression
T f  iK f m
 Freezing point declines relative to molality of the solute
LACC Chem 101
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Vapor Pressure Lowering
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 Similar to mole fraction calc from gas laws
Psolvent   solvent P
 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
LACC Chem 101
Raoult’s Law
PA   A PA 
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 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
LACC Chem 101
Osmotic Pressure
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 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:

RT
RT
ln   A  A   
ln   A 
V
V
 If we assume an ideal solution at low concentration, we can
further approximate:
  iMRT
LACC Chem 101
OSMOSIS
Osmosis is the phenomenon of solvent flow through a 13
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.
LACC Chem 101
Lecture Questions:
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1. At 25ºC, the vapor pressure of C6H6 is 0.1252 atm. When 10.00 g of
an unknown volatile substance is dissolved in 100.0 g of
benzene, the vapor pressure of the solution, at 25ºC, is 0.1199
atm. Calculate the molar mass of the solute.
2. What is the osmotic pressure at 25º C of an isotonic saline
solution that contains 0.900 g NaCl in 100 mL of aqueous
solution? Assume i is ideal.
3. At 25ºC, the freezing point of a NaCl aqueous solution is -0.406ºC.
Calculate the osmotic pressure this solution has on a semipermeable membrane if the concentration of the solution is
equivalent to the molality.
LACC Chem 101
Workshop on Colligative Properties
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1. Determine the vapor pressure of a solution of 92.1 g of glycerin,
C3H5(OH)3, in 184.4 g of ethanol at 40 C. The vapor pressure of pure
ethanol is 0.178 atm at 40 C, and glycerin is essentially nonvolatile.
2. Find the boiling point of a solution of 92.1 g of iodine in 800.0 g of
chloroform.
3. Calculate the freezing point of a solution of 0.724 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 0.750
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: 0.050 m CaCl2, 0.15 m NaCl, 0.10 m HCl, 0.050 m
HC2H3O2, and 0.10 m C12H22O11.
LACC Chem 101
Workshop continued:
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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?
7. 0.500 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 220.0 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 0.090 g of this compound in 1.10 g of
camphor melts at 158.4 C. The melting point of pure camphor is
178.4 C, and its freezing point depression constant is 37.7 C m-1.
What is the molecular formula of the solute?
LACC Chem 101
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
LACC Chem 101
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Types of COLLOIDS
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Continous
Phase
Dispersed
Phase
Name
Example
Gas
Gas
Liquid
Liquid
Liquid
Solid
Gas
Liquid
Aerosol
Aerosol
Foam
Emulsion
Liquid
Solid
Sol
Solid
Gas
Foam
Solid
Liquid
Gel
Solid
Solid
Solid sol
Fog, mist
Smoke
Whipped Cream
Mayonnaise
(oil dispersed
in water
AgCl(s) dispersed
in H2O
Pumice, plastic
foams
Jelly, Opal
(mineral with
liquid inclusions)
Ruby glass
(glass with
dispersed metal)
LACC Chem 101
Types of Colloids
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 Hydrophilic Colloid
 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
LACC Chem 101
Association Colloid
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 Micelle
 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)
LACC Chem 101
The End
 End of Chem 101
 Final Wednesday May 29 2013
 150 points
LACC Chem 101
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