Untitled - Kelly Walsh High School

Solutions 179
Just as the freezing point of a solution is always lower than the pure solvent, the
boiling point of a solution is always higher than the solvent. The relationship is
similar to the one for the freezing point depression above and is:
T b iK bm
In this equation, T b is the number of degrees that the boiling point has been
elevated (the difference between the boiling point of the pure solvent and the
solution), K b is the boiling-point elevation constant, m is the molality of the
solute, and i is again the van’t Hoff factor.
Osmotic Pressure
A U-tube contains a solution and pure solvent. A semipermeable membrane separates
the two components. Such a membrane allows the passage of solvent molecules
but not solute particles. This arrangement will result in the level of the solvent
side decreasing while the solution side increasing. This indicates that the solvent
molecules are passing through the semipermeable membrane. This process is
osmosis. Eventually the system would reach equilibrium and the difference in levels
would remain constant. The difference in the two levels is related to the osmotic
pressure. In fact, one could exert a pressure on the solution side exceeding the
osmotic pressure. This will cause the solvent molecules to move back through the
semipermeable membrane into the solvent side. This process is reverse osmosis
and is the basis of desalination of seawater for drinking purposes.
The osmotic pressure is a colligative property and mathematically represented as:
(nRT/V) i iMRT
In this equation, is the osmotic pressure in atmospheres, n is the number of
moles of solute, R is the ideal gas constant (0.0821 L . atm/Kmol), T is the
Kelvin temperature, V is the volume of the solution and i is the van’t Hoff factor.
If one knows the moles of solute and the volume in liters, n/V may be
replaced by the molarity, M. It is possible to calculate the molar mass of a solute
from osmotic pressure measurements. This is especially useful in the determination
of the molar mass of large molecules such as proteins.
12-4 Colloids
Particles will settle out of water from a muddy stream. This water is a heterogeneous
mixture, where the particles are large (in excess of 10 3 nm in diameter)
and is a suspension. On the other hand, dissolving sodium chloride in water produces
a true homogeneous solution, where the solute particles are less than
1 nm in diameter. Particles do not settle out of a true solution because of their
very small particle size. However, there is a mixture with particle diameters