Colligative Properties Consider three beakers: 50.0 g of ice 50.0 g of ice + 0.15 moles NaCl 50.0 g of ice + 0.15 moles sugar (sucrose) What will the temperature

Colligative Properties

Consider three beakers: 50.0 g of ice 50.0 g of ice + 0.15 moles NaCl 50.0 g of ice + 0.15 moles sugar

(sucrose)

What will the temperature of each beaker be? Beaker 1: Beaker 2: Beaker 3:


The reduction of the freezing point of a substance is an example of a colligative property: A property of a solvent that depends

on the total number of solute particles present

There are four colligative properties to consider: Vapor pressure lowering (Raoult’s

Law) Freezing point depression Boiling point elevation Osmotic pressure

Colligative Properties – Vapor Pressure

A solvent in a closed container reaches a state of dynamic equilibrium.

The pressure exerted by the vapor in the headspace is referred to as the vapor pressure of the solvent.

The addition of any nonvolatile solute (one with no measurable vapor pressure) to any solvent reduces the vapor pressure of the solvent.


Nonvolatile solutes reduce the ability of the surface solvent molecules to escape the liquid. Vapor pressure is reduced.

The extent of vapor pressure lowering depends on the amount of solute. Raoult’s Law quantifies the amount of

vapor pressure lowering that is observed.


Raoult’s Law:PA = XAPO

A

where PA = partial pressure of the solvent (A) vapor above the solution (ie with the solute)

XA = mole fraction of the solvent (A)

PoA = vapor pressure of the pure

solvent (A)


Example: The vapor pressure of water at 20oC is 17.5 torr. Calculate the vapor pressure of an aqueous solution prepared by adding 36.0 g of glucose (C6H12O6) to 14.4 g of water.


Answer: 14.0 torr


Example: The vapor pressure of pure water at 110oC is 1070 torr. A solution of ethylene glycol and water has a vapor pressure of 1.10 atm at the same temperature. What is the mole fraction of ethylene glycol in the solution?

Both ethylene glycol and water are liquids. How do you know which one is the solvent and which one is the solute?


Answer: XEG = 0.219


Ideal solutions are those that obey Raoult’s Law.

Real solutions show approximately ideal behavior when: The solution concentration is low The solute and solvent have similarly

sized molecules The solute and solvent have similar

types of intermolecular forces.


Raoult’s Law breaks down when solvent-solvent and solute-solute intermolecular forces of attraction are much stronger or weaker than solute-solvent intermolecular forces.

Colligative Properties – BP Elevation

The addition of a nonvolatile solute causes solutions to have higher boiling points than the pure solvent.

Vapor pressure decreases with addition of non-volatile solute.

Higher temperature is needed in order for vapor pressure to equal 1 atm.

Colligative Properties- BP Elevation

The change in boiling point is proportional to the number of solute particles present and can be related to the molality of the solution:

Tb = Kb.m

where Tb = boiling point elevationKb = molal boiling point elevation constantm = molality of solution

The value of Kb depends only on the identity of the solvent (see Table 13.4).

Colligative Properties - BP Elevation

Example: Calculate the boiling point of an aqueous solution that contains 20.0 mass % ethylene glycol (C2H6O2, a nonvolatile liquid).

Solute = Solvent =Kb (solvent) =

Tb = Kb m


Molality of solute:

Tb =

BP = 102.1oC


Example: The boiling point of an aqueous solution that is 1.0 m in NaCl is 101.02oC whereas the boiling point of an aqueous solution that is 1.0 m in glucose (C6H12O6) is 100.51oC. Explain why.


Example: A solution containing 4.5 g of glycerol, a nonvolatile nonelectrolyte, in 100.0 g of ethanol has a boiling point of 79.0oC. If the normal BP of ethanol is 78.4oC, calculate the molar mass of glycerol.

Given: Tb =mass solute =mass solvent =Kb = 1.22oC/m (Table 13.4)

Find: molar mass (g/mol) Tb = Kb m


Step 1: Calculate molality of solution

Step 2: Calculate moles of solute present

Step 3: Calculate molar mass

Colligative Properties - Freezing Pt Depression

Freezing point of the solution is lower than that of the pure solvent.

The addition of a nonvolatile solute causes solutions to have lower freezing points than the pure solvent.

Solid-liquid equilibrium line rises ~ vertically from the triple point, which is lower than that of pure solvent.


The magnitude of the freezing point depression is proportional to the number of solute particles and can be related to the molality of the solution.

Tf = Kf mwhere Tf = freezing point depression

Kf = molal freezing point depression constant

m = molality of solution

The value of Kf depends only on the identity of the solvent (see Table 13.4).


Example: Calculate the freezing point depression of a solution that contains 5.15 g of benzene (C6H6) dissolved in 50.0 g of CCl4.

Given: mass solute =mass solvent =Kf solvent =

Find: Tf

Tf = Kf m


Molality of solution:

Tf =


Example: Which of the following will give the lowest freezing point when added to 1 kg of water: 1 mol of Co(C2H3O2)2, 2 mol KCl, or 3 mol of ethylene glycol (C2H6O2)? Explain why.


Reminder: You should be able to do the following as well:

Calculate the freezing point of any solution given enough information to calculate the molality of the solution and the value of Kf

Calculate the molar mass of a solution given the value of Kf and the freezing point depression (or the freezing points of the solution and the pure solvent).

Colligative Properties - Osmosis

Some substances form semipermeable membranes, allowing some smaller particles to pass through, but blocking other larger particles.

In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so.

If two solutions with identical concentration (isotonic solutions) are separated by a semipermeable membrane, no net movement of solvent occurs.


Osmosis: the net movement of a solvent through a semipermeable membrane toward the solution with greater solute concentration.

In osmosis, there is net movement of solvent from the area of lower solute concentration to the area of higher solute concentration. Movement of solvent from high solvent

concentration to low solvent concentration


Osmosis plays an important role in living systems: Membranes of red blood

cells are semipermeable.

Placing a red blood cell in a hypertonic solution (solute concentration outside the cell is greater than inside the cell) causes water to flow out of the cell in a process called crenation.


Placing a red blood cell in a hypotonic solution (solute concentration outside the cell is less than that inside the cell) causes water to flow into the cell. The cell ruptures in a process called

hemolysis.


Other everyday examples of osmosis:

A cucumber placed in brine solution loses water and becomes a pickle.

A limp carrot placed in water becomes firm because water enters by osmosis.

Eating large quantities of salty food causes retention of water and swelling of tissues (edema).

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Colligative Properties Consider three beakers: 50.0 g of ice 50.0 g of ice + 0.15 moles NaCl 50.0 g of ice + 0.15 moles sugar (sucrose) What will the temperature