Organic chemistry and Biological chemistry for Health Sciences
59-191
Lecture 4
COLLIGATIVE PROPERTIES:
Properties of solutions or colloidal dispersions that depend only on the number of solute
particles per unit quantity of solvent, not on the chemical identities of the solutes, are
called colligative properties.
Examples:



Depression of freezing point
Elevation of boiling point
Osmosis/dialysis- abilities of components of a solution to migrate through certain
kinds of membrane
1 mole of NaCl in 100 g of water and another solution with one mole KBr in 100 g water,
they both freeze at the same temperature, at –3.40C and they both boil at 1010C.
Cells in living systems are enclosed by cell membranes. On both sides of the membrane
are aqueous systems holding substances both in solutions and colloidal dispersion.
Two factors control the movement through membranes
Active transport

Dialysis
Active transport:

Movement of ions and molecules through membranes by endothermic chemical
reactions
 Ions are pumped against concentration gradient
Dialysis:
Dialysis depends on the semipermeable nature of a cell membrane.
A semipermeable membrane is one that is able to let some but not all kinds of ions and
molecules pass through it.
Cellophane is an example of synthetic semipermeable membrane.
When cellophane separates water from a dilute solution that also contain colloidal size
particles, like starch molecules, for example, only water molecules and other small
molecules and ions can migrate through it.
Membrane with this selectivity is called dialyzing membrane.
A semipermeable membrane that is so selective that only the solvent molecules can get
through, is called an osmotic membrane. The effective sizes of ions which are expanded
by solvent cages are too large for the pores.
OSMOSIS:
Osmosis is the net migration of water through an osmotic membrane from a solution of
low concentration of solute to a solution with a higher concentration of solute.
Osmotic pressure:

The exact back pressure necessary to prevent osmosis is called the osmotic pressure
of the solution.

It is the potential pressure that can be realized only when an osmotic membrane
separates the solution from pure water.
The value of osmotic pressure is directly proportional to the molar concentration of the
solute particles in the solution.
V = nRT ( is osmotic pressure)
 = n/V. RT
=MRT (M = n/V = mol/L)
Solute particles can be ions, molecules, colloids etc.
 is a colligative property.
Ions of ionic compounds individually affect osmotic pressure:
When solute is an ionic compound, like NaCl, the concentration of particles-ions in this
case is twice the molar concentration of the salt itself.
0.1M NaCl = 2x 0.1 M = 0.2 mols ions/litre
0.1M Na2SO4 = 3X 0.1M = 0.3 mol ions/litre
So 0.1M NaCl will exert twice as much osmotic pressure as 0.1 M Glucose solution.
So in order to reveal enough about solutions osmotic pressure another concentration
expression is introduced, called Osmolality, symbolized as Osm.
Osmolarity of 0.1M NaCl is 0.2 mol/L
Dialysis:

Dialysis is like osmosis, but membrane is more permeable.

Not only water molecules but also ordinary sized ions and molecules move through
the membrane

Dialyzing membrane can be thought of as having larger pore than osmotic membrane.
F.Example
If a solution of protein is separated from a bathing solution by a semipermeable
membrane, small molecules and ions can pass through the semipermeable membrane to
equilibrate between the protein solution and the bathing solution, called the dialysis bath
or dialysate.
Dialysis of bloodstream:
The body tries to maintain the concentrations of all the substances that circulate in blood
within fairly narrow limit.
Quite complicated mechanisms exist to excrete or retain solutes or to retain or excrete
water.
When they fail, the consequences can be life threatening.
Shock syndrome:

Dramatic increase in the permeability of blood capillaries to colloidal size particles

Decrease of colloidal substances in blood

Blood become less concentrated and less able to take up water from spaces surround
the capillaries


Sufficient decrease in colloidal osmotic pressure results in net loss of water from
blood
Blood volume decreases


Makes more difficult to bring nutrient to brain and carry wastes away
Brain function less well and the result to the nervous system is called shock