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Colligative Properties. LACC Chem101. 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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colligative properties1
Colligative Properties

LACC Chem 101

  • 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
colligative properties continued
Colligative Properties (continued)

LACC Chem 101

  • 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 αthat dissociates
boiling point elevation
Boiling Point Elevation

LACC Chem 101

  • 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!!!
boiling point elevation1
Boiling Point Elevation

LACC Chem 101

At Low Concentrations

For very dilute solutions:


LACC Chem 101

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

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?

workshop on boiling point elevation
Workshop on Boiling Point Elevation

LACC Chem 101

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?

freezing point depression
Freezing Point Depression

LACC Chem 101

Freezing point declines relative to molality of the solute

vapor pressure lowering
Vapor Pressure Lowering

LACC Chem 101

  • 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
raoult s law
Raoult’s Law

LACC Chem 101

  • 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
osmotic pressure
Osmotic Pressure

LACC Chem 101

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:



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


LACC Chem 101


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.

LACC Chem 101


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 mNaCl, 0.10 mHCl, 0.050 m HC2H3O2, and 0.10 m C12H22O11.

LACC Chem 101


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?

LACC Chem 101


LACC Chem 101

  • 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


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


Solid Liquid Gel Jelly, Opal

(mineral with

liquid inclusions)

Solid Solid Solid sol Ruby glass

(glass with

dispersed metal)

LACC Chem 101

types of colloids
Types of Colloids

LACC Chem 101

  • 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
association colloid
Association Colloid

LACC Chem 101

  • 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)
the end
The End

LACC Chem 101

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