Page 114 - Instant notes

Physical chemistry 100

where T b and are the boiling points of solvent A (with B added) and pure solvent A,
respectively, and is the standard enthalpy of vaporization of solvent A (the
enthalpy required to boil a mole of liquid).
The change in sign of the expression means that adding solute B stabilizes liquid A
and leads to an increase in the boiling point of the solvent, which corresponds to a
positive value of , which is known as the boiling point elevation. The
final expression is:

is the ebullioscopic constant or the boiling point elevation constant. As with the
cryoscopic constant, this is a constant for any given solvent, and values have been
measured experimentally and tabulated, which again allows the determination of x B and
m B for any solute from ∆T b.

Osmotic pressure

Osmosis is the movement of a solvent from a solution of lower solute concentration
(higher solvent concentration) to one of higher solute concentration (lower solvent
concentration). In osmotic pressure measurements, a semi-permeable membrane, which
is permeable only to solvent (as it has holes that are small enough to prevent large solute
molecules passing through) separates two liquids. This means that this technique is only
used for relatively large solute molecules, often polymers or biological macromolecules.
Typically, one liquid is pure water and the other is the solute solution of interest. This
produces a flow of water from the solvent to the solute solution. The experimental
apparatus (Fig. 1) incorporates two identical vertical columns, one for each liquid, and
the height of the liquid in the solution column increases relative to that in the pure solvent
column due to this net flow.
The extra height (and mass of water) in the solute column compared to the solvent
column produces an excess gravitational force (and pressure) in the solute compartment.
This produces an opposing pressure to the osmotic pressure, trying to squeeze solvent
back through the membrane. When this pressure exactly opposes the osmotic pressure,
Π, the pressure driving osmotic solvent flow, the flow ceases and an equilibrium is
established with the heights of the liquid in the two columns remaining constant. The
pressure exerted at the foot of a column of solvent is proportional to its height, so a
reading of the difference in heights of the two columns then leads directly to the
difference in pressure, which is itself equal to Π. The osmotic pressure of an ideal-dilute
solution is then given by: