Colligative Properties

Question 1

What is a colligative property?

Answer 1

A physical property of dilute solutions that depends on the number of non-volatile solute particles present in the solution rather than their chemical nature

Question 2

What are examples of colligative properties?

Answer 2

1. Boiling point elevation
2. Freezing Point depression
3. Vapor pressure lowering
4. Osmotic pressure

Question 3

What are the limitations of colligative properties?

Answer 3

1. The solute should not associate or dissociate in solvent.
   - Association decreases the number of particles, lowering the colligative property and causing increased relative molecular mass of the solute.
   - Dissociation increases the number of particles and the value of colligative property becomes higher resulting into reduced relative molecular mass of the solute
2. The solution should be dilute to approximate to an ideal solution
3. The solute should be non-volatile so that only solvent molecules vaporize and exert vapor pressure onto the solution
4. There should not be a chemical reaction between the solute and the solvent

Question 4

Define boiling point of a liquid

Answer 4

This is the constant temperature at which the liquid changes into gas when its saturated vapor pressure is equal to the atmospheric pressure

Question 5

What is the effect of a non-volatile solute on the boiling point of a liquid?

Answer 5

• boiling takes place when the saturated vapor pressure of a liquid is equal to the external atmospheric pressure
• Dissolving a non-volatile solute in a given solvent gives a solution whose boiling point is greater than that of the pure solvent.

Question 6

Why does dissolving a non-volatile solute in a given solvent give a solution whose boiling point is greater than that of the pure solvent?

Answer 6

• The non-volatile solute particles tend to cover the surface of the solution thereby reducing the escaping tendency of solvent molecules into vapor state.
• This lowers the vapor pressure above the liquid.
• The solution must be heated to a higher temperature in order to raise enough saturated vapor pressure equal to the atmospheric pressure for boiling of the solution to take place.

Question 7

Define boiling point elevation constant (ebullioscopic constant).

Answer 7

This is the amount by which the boiling point of 1000g of a pure solvent is increased by dissolving one mole of a non-volatile solute into the solvent

Question 8

True or false
At constant pressure boiling point elevation is directly proportional to the mass of the solute

Answer 8

True

Question 9

True or false
At constant pressure boiling point elevation is inversely proportional to the relative molecular mass of the solute.

Answer 9

True

Question 10

Describe the procedure of the experiment to determine molecular mass of a non-volatile solute by boiling point elevation method.

Answer 10

1. Small mass (Wg) of pure solvent is placed in a graduated vessel
2. The pure boiling solvent vapor is passed through it until the solvent boils at a constant temperature (T1)
3. Excess vapor passes through a small hole in the bulbous part of the apparatus and can be condensed and collected for reuse
4. Excess solvent is removed from the graduated vessel so as to keep the volume of solvent at its original level
5. A small weighed mass (mg) of non-volatile solute is dissolved in remaining volume to form a dilute solution
6. Pure boiling solvent vapor is passed through the dilute solution until it boils at a constant temperature (T2) measured using a Beckman’s thermometer and recorded

Question 11

Define freezing point of a liquid

Answer 11

This is the temperature at which a liquid changes into a solid when the two phases are in equilibrium with each other and have equal saturated vapor pressure

Question 12

Explain why dissolving a non-volatile solute in a pure solvent lowers the freezing point of the solvent

Answer 12

• molecules of the non-volatile solute at the surface of the solution reduce the escaping tendency of the volatile solvent molecules into vapor state
• Fewer solvent vapor molecules above the solution results into lower vapor pressure exerted onto the solution compared to the pure solvent
• Saturated vapor pressure of the solution and crystallized solid will only be equal at lower temperature than that of the pure solvent
• The solid-liquid equilibrium is only possible at a temperature lower than that of the freezing point of the pure solvent resulting into the lower freezing point for the solution

Question 13

Define freezing point depression (Cryoscopic constant)

Answer 13

This is the amount by which the freezing point of 1000 g of a pure solvent is lowered by dissolving one mole of a non-volatile solute into the solvent

Question 14

Describe the process of determining the RMM of a non-volatile solute by freezing point depression method

Answer 14

• A solvent of known mass (Wg) and freezing point constant (Kf) is placed in the inner glass tube fitted with a Beckmann thermometer and stirrer.
• The inner tube is placed in an air-jacket to ensure that temperature drops slowly. The air- jacket is then immersed in an ice-salt freezing mixture.
• The solvent and the freezing mixture are constantly stirred and the constant temperature (T1) at which the solvent freezes is measured using a Beckmann’s thermometer and recorded.
• The inner glass tube is removed and the solvent melted by slightly warming it.
• A known mass (mg) of the non-volatile solute is added to molten solvent through the side arm and the
  mixture stirred to dissolve it.
• The inner glass vessel is put back in the air-jacket and the whole setup placed back into the freezing
  mixture.
• The solution and the freezing mixture are stirred constantly and constant temperature (T2) at which the
  solution freezes is measured using a Beckmann’s thermometer and recorded.

Question 15

Using cooling curves of the solvent and solution, describe how one would determine freezing point depression and RMM of a non-volatile solute

Answer 15

• A known mass (W g) of a pure solvent is placed in the inner glass tube which is then placed in a freezing mixture.
• The solvent is stirred continuously and its temperature recorded after known intervals of time until crystallization occurs
• The glass tube is removed from the freezing mixture and solid solvent melted by warming the glass tube.
• A known mass (m g) of a non-volatile solute is added to the solvent, the mixture stirred to dissolve the
  solute and whole set up placed back in the freezing mixture.
• The freezing mixture is constantly stirred and the temperature of the solution is recorded at the same
  regular time intervals until enough crystals appear in the solution.
• On the same axes, graphs of temperature against time are plotted for both the pure solvent and solution.

Question 16

What does Raoult’s law of vapor pressure lowering state?

Answer 16

The relative lowering of vapour pressure of a pure solvent due to a non-volatile solute dissolved in it is equal to the mole fraction of the solute in the solution at a particular temperature

Question 17

Define osmosis

Answer 17

The flow of solvent molecules from a less concentrated solution to a more concentrated solution of the same solvent across a semi-permeable membrane.

Question 18

What is a semi-permeable membrane?

Answer 18

A substance that allows solvent molecules to pass through it but not solute molecules

Question 19

Define osmotic pressure

Answer 19

The pressure that must be applied to a solution to prevent osmosis from taking place when the solution is separated from its pure solvent by a semi-permeable membrane.

Question 20

What are the laws of osmotic pressure?

Answer 20

• The osmotic pressure of a solution is directly proportional to its concentration if temperature is kept constant
• The osmotic pressure of a solution of a fixed concentration is directly proportional to absolute temperature

Question 21

What is the value of R in osmotic pressure calculations?

Answer 21

8.314 JKmol

Question 22

True or false
In osmotic pressure calculations;
Either Nm or atm may be used without changing one to the other

Answer 22

True
The only changes that may occur is when osmotic pressure in kPa or mmHg

Units to be used;
Osmotic pressure => Nm, atm (no change), kPa (change to Pa=Nm), mmHg (divide by 760mmHg)

Volume=> per meter cubed or I dm3

Temperature => Kelvins

Mass=> grams

n= mg/ Mr

Question 23

Describe the procedure for an experiment to determine the RMM of a non-volatile solute by osmotic pressure method

Answer 23

• The porous pot is filled with the solvent until it reaches a known level X in the capillary tube.
• The dilute solution under investigation is placed in the outer jacket and the whole apparatus put in thermostat to keep temperature constant at a fixed value T.
• The solvent molecules are allowed to flow through the semi-permeable membrane into the solution hence forcing the solvent level in the capillary tube to fall below point X.
• Pressure is applied on the s olution to force the solvent molecule move back into the solution until the when the level of the solvent reaches X again in capillary tube.
• The pressure that is just sufficient enough to force the solvent level back to point X in the capillary tube is the osmotic pressure of the solution at temperature, T.
• The osmotic pressure is measured by a pressure gauge.

Question 24

Why is osmotic pressure method preferred to boiling point elevation method in determining RMM of a polymer?

Answer 24

Boiling point elevation is inversely proportional to the RMM of the solution, thus for polymers, the molecular mass is so big that elevation of boiling is so small and cannot be accurately measured using a thermometer leading to inaccurate values of RMM