188 ❯ STEP 4. Review the Knowledge You Need to Score High
The osmotic pressure is a colligative property and mathematically can be represented
as p = (nRT/V) i, where p is the osmotic pressure in atmospheres; n is the number of
moles of solute; R is the ideal gas constant 0.0821 L. atm/K. mol; T is the Kelvin tem-
perature; V is the volume of the solution; and i is the van’t Hoff factor. Measurements
of the osmotic pressure can be used to calculate the molar mass of a solute. This is especially
useful in determining the molar mass of large molecules such as proteins.
For example, a solution prepared by dissolving 8.95 mg of a gene fragment in
35.0 mL of water has an osmotic pressure of 0.335 torr at 25.0°C. Assuming the frag-
ment is a nonelectrolyte, determine the molar mass of the gene fragment.Rearrange p = (nRT/V) i to n = p V/RT (i = 1 for a nonelectrolyte)
=×
×
=×
−−(0.335 torr) (35.0 mL)0.0821
L atm
molK(298.2 K)
1 atm
760 torr1 L
1, 000 mL6.30 10 mol(8.95 mg)(0.001g/mg)
6.30 10 mol1.42 10 g/mol774Colloids
If you watch a glass of muddy water, you will see particles in the water settling out. This
is a heterogeneous mixture where the particles are large (in excess of 1,000 nm), and it
is called a suspension. In contrast, dissolving sodium chloride in water results in a true
homogeneous solution, with solute particles less than 1 nm in diameter. True solutions
do not settle out because of the very small particle size. But, there are mixtures whose
solute diameters fall in between solutions and suspensions. These are called colloids and
have solute particles in the range of 1 to 1,000 nm diameter. Table 13.1 shows some rep-
resentative colloids.
Many times it is difficult to distinguish a colloid from a true solution. The most
common method is to shine a light through the mixture under investigation. A light shone
through a true solution is invisible, but a light shone through a colloid is visible because the
light reflects off the larger colloid particles. This is called the Tyndall effect.Table 13.1 Common Colloid Types
COLLOID TYPE SUBSTANCE DISPERSED DISPERSING MEDIUM EXAMPLES
aerosol solid gas smoke
aerosol liquid gas fog
solid foam gas solid marshmallow
foam gas liquid whipped cream
emulsion liquid liquid milk, mayonnaise
solid emulsion liquid solid cheese, butter
sol solid liquid paint, gelatinKEY IDEA