Physical Chemistry 2.pdf - OER@AVU - African Virtual University

1.9.2 Osmosis
African Virtual University
Osmosis is the flow of material through a semi-permeable membrane in which the
pores allow passage of solvent molecules but are too small for solute molecules. This
process is particularly important in living things where various mixtures and solutions
are separated by membranes. Many plant and animal cell walls are made of semipermeable
membranes. Figure 1.9 illustrates this process. Consider a cell consisting
of two compartments separated by semi-permeable membrane one of which contains
a dilute solution and the other pure water. The liquid in both compartments is set such
that they are at the same level. After some time the liquid in the left compartment
rises whilst the level on the right hand side decreases.
The water molecules from the right migrate to the left whilst the solutes are retained
in the same side. At some point the flow of the solvent ceases due to an opposing
pressure known as osmotic pressure denoted by, π.
cRT
π =
M
where c is the concentration in g L −1 , R is the gas constant, T is the absolute temperature
and M is the molar mass.
Interactive Exercise: Colligative Properties
P + ∏
Solut e molecules
Semi-per meab le
membra ne
P
Solven t molecules
F ig ure 1.9 Diagram
showing the migration of
water m olecules from the
right hand side side. to the left.
The large solu te molecu les
remain on the left hand
side.
Figure 1.9 Diagram showing the migration of water molecules from the right hand
side to the left. The large solute molecules remain on the left hand
Example 3
Albumin is human protein plasma with a molar mass of 69 000 g mol−1 . What would
be the osmotic pressure of a solution of this protein containing 2.0 g per 100 mL at
25 °C. Assume that the experiment is carried out using a salt solution for solvent and
a membrane soluble to the salt.