Cliffs AP Chemistry, 3rd Edition

Figure 2: Freezing Point Graph for Pure Solvent and for Solution

Scenario:A student set up an apparatus to determine the molecular mass through freezing
point depression (see Figure 3)

Figure 3

of naphthalene (see Figure 4). The student measured out 52.0 grams of paradichlorobenzene
and 4.0 grams of naphthalene, placed them in a test tube in a hot-water bath and allowed the
mixture to completely melt. The mixture was stirred well. The tube was then removed from the
hot-water bath, dried and allowed to cool while it was gently stirred to minimize supercooling.
Temperature readings were taken every 60. seconds until the mixture solidified and the temper-
ature stabilized. A graph was drawn of the results (see Figure 5).

Split stopper

Thermometer

Stirrer

Water bath

Iron ring and

wire gauze

Bunsen burner

Temperature vs. Cooling Time for Solvent and Solution

a = freezing point of

pure solvent

b = freezing point of

solution

Pure Solvent

Solution

Supercooling

Time of Cooling

Tfp

T (°C)

a

b

Part III: AP Chemistry Laboratory Experiments