5 Steps to a 5 AP Chemistry

The osmotic pressure is a colligative property and mathematically can be represented as p=

(nRT/V) i,where ois the osmotic pressure in atmospheres; nis the number of moles of

solute; Ris the ideal gas constant 0.0821 L .atm/K.mol; T is the Kelvin temperature; Vis

the volume of the solution; and iis the van’t Hoff factor. Measurements of the osmotic pres-

sure 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 fragment is a non-

electrolyte, determine the molar mass of the gene fragment.

Rearrange p=(nRT/V) ito n=pV/RT(i=1 for a nonelectrolyte)

Colloids

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 1000 nm), and it is called

a suspension. In contrast, dissolving sodium chloride in water results in a true homoge-

neous 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 colloidsand have solute parti-

cles in the range of 1 to 1000 nm diameter. Table 13.1 shows some representative 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 shown 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, gelatin

Experimental

Experimental procedures for solutions fall into two broad categories. One group involves

concentration units, and the other involves colligative properties. In both cases, keeping

(. )(. / )

.

.

8 95 0 001

630 10

7 142 10^4

mg g mg

mol

g/mol

×

− = ×

0 335 35 0

0 0821 298

..

..

torr mL

Latm

mol K

()()

⎛

⎝

⎜

⎞

⎠

⎟^22

11

1000

630.

K

atm

760 torr

L

mL

()

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟=×^110

− 7

Solutions and Colligative Properties  187

KEY IDEA

KEY IDEA

STRATEGY