Colligative Properties

1
Colligative Properties

• Honors ChemistryMrs. Coyle

2
How do you get from this
3
to this?
4
Add an ionic compound!
5
How do properties of solutions differ from
properties of the solute and solvent?

• We will examine four properties called
  colligative properties
• Vapor Pressure
• Freezing Point
• Boiling Point
• Osmotic Pressure

6
Colligative Properties

• Depend on the number of solute particles
  (concentration) and not the type of solute
  particles.
• Ionic solutes have a greater effect on
  colligative properties since they dissociate and
  produce a higher number of particles.

7
Part I

• Vapor Pressure Depression

8
Vapor Pressure Depression

• The vapor pressure of a solution is lower than
  the that of the pure liquid.
• Nonvolatile solute particles inhibit the movement
  of solvent particles into the vapor phase, since
  there is less solvent per unit volume, and so
  reduce the vapor pressure.

9
Phase Diagram of Water (Black Line) and Solution
(Pink Line)
http//www.chemguide.co.uk/physical/phaseeqia/pdso
lution3.gif
10
Raoults Law

• Psolution xsolventPsolvent
• The vapor pressure of a solution varies directly
  as the mole fraction of the solvent particles.
• Tutorial Link
• http//www.wwnorton.com/college/chemistry/gilbert/
  overview/ch9.htmvapor_pressure

11
(No Transcript)
12
Ideal Solution

• A liquid-liquid solution that obeys Raoults Law.

13
Example 1 Nonvolatile Solute

• Find the vapor pressure of a solution
  containing 63.0 g of glycerine(C3H8O3) in 500.0 g
  of water at 55oC. The vapor pressure of water at
  25oC is 3.2 kPa.(Hint Find the vapor pressure at
  55oC. Psolution xsolventPsolvent
• Answer 0.685mol C3H8O3 , 27.8 mol H2O,
  Psolvent3.52kPa, Psolution 3.44kPa

14
Vapor Pressure Depression

• ?P Psolvent-Psolution

15
Example 2 Solution of two volatile substances
(Hint Use Daltons Law of Partial Pressures)

• Find the vapor pressure of a solution
  containing a mixture of 78.0g benzene(C6H6) and
  92.0g toluene (C7H8) at 20oC.
• Pbenzene 75 torr
• Ptoluene 22 torr
• Answer 1 mol C6H6 , 1 mol C7H8, 37.5 torr C6H6,
  11.0 torr C7H8, PT 48.5 torr

16
Example 3 Ionic Solute

• Calculate the vapor pressure of a solution
  containing 95.3g MgCl2 and 180g water at 25oC.
  The vapor pressure of pure water at 25oC is
  3.2kPa. (Hint Each formula unit of MgCl2
  produces 3 particles when dissolved) Psolution
  xsolventPsolvent
• Answer 3 mol of MgCl2 particles, 2.46kPa

17
Part II

• Freezing Point Depression
• Boiling Point Elevation

18
Freezing and Boiling Point

• When a non volatile solute is added to a solvent
  the result is usually
• Boiling Point Elevation
• Freezing Point Depression

19
Boiling Point Elevation
Addition of a nonvolatile solute lowers the
vapor pressure of the solvent and therefore
raises the boiling point.
20
Phase Diagram of Water (Black Line) and Solution
(Pink Line)
http//www.chemguide.co.uk/physical/phaseeqia/pdso
lution3.gif
21
Boiling Point Elevation

• ?Tb m. kb
• Tsolution Tb ?T
• kb molal boiling point constant
• m molality of the solute particles
• For water kb 0.512oC/m

22
Note

• If the solute is ionic, dissociation occurs.
• More particles in solution.
• Ionic solutions have greater effect on
  colligative properties.

23
Freezing Point Depression

• Addition of a nonvolatile solute reduces the
• vapor pressure of ice and therefore reduces the
• freezing point.

24
Freezing Point Depression
25
Phase Diagram of Water (Black Line) and Solution
(Pink Line)
http//www.chemguide.co.uk/physical/phaseeqia/pdso
lution3.gif
26
Example

• Salt in water to melt ice by reducing the
  freezing point of ice.

27
Example

• Addition of ethylene glycol C2H6O2 (antifreeze)
  to the water in the radiator of a car allows the
  radiator to have a broader range of operation by
  raising the boiling point and lowering the
  freezing point.

28
Freezing Point Depression

• ?Tf m. kf
• Tsolution Tf - ?T
• Kf molal freezing point constant
• m molality of the solute particles
• Example for water kf 1.86oC/m

29
Example 4

• Find the boiling point of a solution containing
• 57.0 g of sugar (MM 342g/mol) dissolved in
• 50.0 g of water.
• Answer101.7oC

30
Example 5

• What is the freezing point of a solution
  containing 100. g of glucose in 150. g of water?
• Answer-6.88oC

31
Example 6 Ionic Solute

• What is the freezing point of a solution
  containing 30.0 g of magnesium chloride in 100. g
  of water?
• Answer-17.6oC

32
Example 7 Calculate Molar Mass of Solute

• 500g of a molecular solute were dissolved in
  250g of water. This resulted in a boiling point
  elevation of 2 K. Calculate the molar mass of the
  solute substance. (kb water 0.51 K kg/mol)
• Answer 510g/mol