Colligative Properties

Question 1

What do colligative properties depend on?

Answer 1

The number of solute particles in solution

Includes osmotic pressure, vapour pressure lowering, boiling point elevation and freezing point depression

Question 2

What is a molar solution?

Answer 2

A one molar (1M) solution is one gram-molecular weight of substance dissolved to a final volume of 1000 ml (qs)

Question 3

What is a molal solution?

Answer 3

A one molal (1m) solution is a one gram-molecular weight of substance dissolved in 1000 g of water (this is not temperature dependant)

Question 4

What is a normal solution?

Answer 4

A one normal (1N) solution is the equivalent weight (one gram-molecular weight/valence) of substance dissolved in water to a final volume of 1000 ml (qs)
Note: the term Normal Saline refers to a 0.9% solution of NaCl

Question 5

What is osmotic pressure?

Answer 5

Consider two solutions separated by a semi-permeable membrane. Solvent will diffuse through membrane to the more concentrated solution (osmosis)
Semi-permeable means only solvent molecules pass through the membrane
Osmotic pressure is the amount of pressure which must be applied to the more concentrated side to stop the passage of solvent molecules

Question 6

What is boiling point?

Answer 6

Boiling point is the temperature at which vapour pressure of a liquid is equal to the external atmospheric pressure of 760 mmHg

Question 7

What happens to the boiling point if a non-volatile substance is added?

Answer 7

Since the vapour pressure is lowered when a non-volatile solute is added, the solution must be brought to a higher temperature than the pure solvent to reach the boiling point

Question 8

What is freezing point?

Answer 8

The freezing point is the temperature where solid and liquid phases of a solution are in equilibrium at a pressure of one atmosphere

Question 9

Identical molal solutions of non-electrolytes all have the same colligative properties. Explain:

Answer 9

19.8 g of dextrose, 18.2 g of mannitol or 9.2 g of glycerin each dissolved in 1000 g of water are all 0.1 molal and contain the same number of particles (molecules)

Question 10

Do identical molal solutions of electrolytes all have the same colligative properties?

Answer 10

No. It is different for electrolytes as these will dissociate and ionize
Each ion will act as a particle so the colligative properties will be a function of the degree of ionization.
Potentially, when in solution, one molecule of NaCl will generate two particles, Na2SO4 will generate three particles and Na3citrate will generate four particles

Question 11

What are millimoles?

Answer 11

SI expresses electrolyte concentration in millimoles per litre (mmol/L) and a mole is the molecular weight of a substance in grams; a millimole is 1/1000 of the molecular weight in grams

Question 12

What are milliequivalents (mEq)?

Answer 12

The equivalent weight (Eq) is the molecular weight of the substance divided by the valence. A milliequivalent (mEq) is 1/1000 of the equivalent weight

Question 13

What are milliosmoles (mOsmol)?

Answer 13

A unit to express osmotic pressure is the milliosmole
For a non-electrolyte like dextrose, 1 mol represents 1 mOsmol
For electrolytes the total number of particles depends on the degree of dissociation
Assuming 100% inoization, 1 mmol of NaCl represents 2 mOsmols of total particles, a mmol of CaCl2 represents 3 miasmal of total particle and 1 mmol of sodium citrate represents 4 mOsmol

Question 14

Why is isotonicity important?

Answer 14

For ophthalmic, nasal, vaginal and parenteral administration, isotonic solutions are better tolerated than those at extremes of hypo- or hypertonicity
We can prepare isotonic solutions using data derived from the colligative properties of the solution

Question 15

How can we use colligative properties to determine isotonicity?

Answer 15

Serum freezes at -0.52ºC so we can use the freezing point depression as a way of determining how much of a substance dissolved in 1000g of water will give an isotonic solution. We saw that a solution of 55.5 g of dextrose monohydrate in 1000g of water had a freezing point depression of 0.52ºC and so a solution containing 5% dextrose monohydrate is isotonic

Question 16

What is the dissociation factor?

Answer 16

Calculations we do to prepare isotonic solutions are approximations but work quite adequately. 
An 'ideal solution' of a  strong electrolyte like sodium chloride will ionize to give two particles but the solutions we deal with are non-ideal since inter-particulate interactions occur
With increasing concentration, the degree of dissociation decreases and we need to use a dissociation factor to account for this
Dissociation factor (i) is also known as van't Hoff's factor

Question 17

What is Volume Adjustment (White-Vincent)?

Answer 17

A method based on determining the quantity of water needed to make an isotonic solution for a given amount of drug followed by dilution with an isotonic solution to make up the volume. It is useful if the final product is to be buffered

Question 18

How is Volume Adjustment applied?

Answer 18

With this method, the weight of the drug is multiplied by its sodium chloride equivalent to obtain the quantity of sodium chloride osmotically equivalent to weight per gram of drug