Colligative Properties

1
Colligative Properties

• LACC Chem101

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 (Raoults Law)
• Osmotic Pressure

3
Colligative Properties (continued)

• Properties of a solvent that depend on total
  concentration of the solute
• Vant 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 a that dissociates

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
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!!!

5
Boiling Point Elevation

• At Low Concentrations
• For very dilute solutions

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. 154.2 g/mol)

7
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?

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 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?

9
Freezing Point Depression

• Freezing point declines relative to molality of
  the solute

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 Raoults Law

11
Raoults 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

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

13
OSMOSIS
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.
14

• Lecture Questions
• 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.
• 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 semi-permeable
  membrane if the concentration of the solution is
  equivalent to the molality.

15
Workshop on Colligative Properties 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.
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? 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?
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

18
Types of COLLOIDS
Continous Dispersed Name Example Phase
Phase
Gas Liquid Aerosol Fog, mist Gas Solid Ae
rosol Smoke Liquid Gas Foam Whipped
Cream Liquid Liquid Emulsion Mayonnaise
(oil dispersed in water Liquid Solid Sol
AgCl(s) dispersed in H2O Solid Gas Foa
m Pumice, plastic foams Solid Liquid Ge
l Jelly, Opal (mineral
with liquid inclusions) Solid Solid Soli
d sol Ruby glass (glass with
dispersed metal)
19
Types of Colloids

• 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

20
Association Colloid

• 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)

21
The End

• End of Chem 101
• Final Wednesday May 29 2013
• 150 points