What does colligative properties depend on?
- depend on a number of solute particles in a solution
- includes osmotic pressure, vapour pressure lowering, boiling point elevation, and freezing point depression
- our discussion will generally deal with dilute solutions (<5% w/v)
What is the difference between a molar solution and a molal solution?
- 1 molar solution (1M)- one gram molecular weight of substance dissolved to a final volume of 1000 mL (qs)
- 1 molal solution (1m)- one gram molecular weight of substance dissolved in 1000 g of water (this one is not temperature sensitive)
What is a normal solution?
- a one normal (1N) solution is the equivalent weight (1 gram molecular weight/valence) of substance dissolved in water to a final volume of 1000 ml (qs)
What does the term normal saline refer to?
- refers to a 0.9% solution of NaCl
What is osmotic pressure?
- 2 solutions separated by a semi-permeable membrane
- solvent will diffuse through the 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
- these are important in biological systems since biological membranes are semi-permeable
What is vapour pressure lowering?
- effect of solute concentration on vapour pressure expressed in Raoult’s law
pa = p0 xa a
xa + xb = 1 xa = 1 – xb
pa = p0 (1 - x ) - where pa is the vapour pressure of the solution, pao is the vapour pressure of the pure solvent, xa is the mole fraction of the solvent and xb is the mole fraction of the solute
– the vapour pressure of pure water is 17.54 mm Hg at 20 degrees celsius
What is the boiling point?
the temperature at which vapour pressure of a liquid is equal to the external atmospheric pressure of 760 mm Hg
What happens to the vapour pressure when a non-volatile solute is added?
the vapour pressure is LOWERED
- the solution must be brought to a higher temperature than the pure solvent to reach the bp(see slide 10)
What is the freezing point?
Freezing point is the temperature where the solid and liquid phases of a solution are in equilibrium at a pressure of 1 atmosphere (see slide 13)
Identical molal solutions of non-electrolytes all have the same ____. How are electrolytes different from this?
colligative properties
electrolytes dissociate and ionize - each ion will act as a particle so the colligative properties will be a function of the degree of ionization
– potentially in solution one molecule of NaCl will generate 2 particles, Na2SO4 will generate 3 particles etc
What does millimoles express?
- SI expresses electrolyte concentration in millimoles oer liter (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
What do milliequivalents express?
- milliequivalents are 1/1000 of the equivalent weight. The equivalent weight (Eq) is the molecular weight of the substance divided by the valence
What are milliosmoles?
- the unit that is used to express osmotic pressure
– for a non-electrolyte - 1 mol represents 1 mOsmol - for electrolytes, the number of particles depends on the degree of dissociation
( example: 1 mol of CaCl2 represents 3 mOsmols)
What happens to electrolyte dissociation as the concentration increases?
electrolyte dissociation is often less than 100% (there are more interactions between solute particles)
- usually the actual or measures osmolarity is less than the calculated one
What routes of administration are isotonic solutions better tolerated with compared to those at extremes of hypo- or hypertonicity?
- ophthalmic, nasal, vaginal and parenteral administration
What is the dissociation factor?
- calculations we do to prepare isotonic solutions
- with increasing concentration, the degree of dissociation decreases and we need to use a dissociation factor to account for this
- dissociation factor- known as the van’t Hoff’s factor
