FreezingPoint_aw update.pdf - Using Freezing-Point...

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Using Freezing-Point Depression to Find Molecular Weight
When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the
number of moles of solute added. This property, known as freezing-point depression, is a
colligative property ; that is, it depends on the ratio of solute and solvent particles, not on the
nature of the substance itself. The equation that shows this relationship is:
 T = K
f
• m
where  T is the freezing point depression, K
f
is the freezing point depression constant for a
particular solvent (3.9°C-kg/mol for lauric acid in this experiment 1 ), and m is the molality of the
solution (in mol solute/kg solvent).
OBJECTIVES
In this experiment, you will

Determine the freezing temperature of pure lauric acid.

Determine the freezing temperature of a solution of benzoic acid and lauric acid.

Calculate the freezing point depression of the mixture

Calculate the experimental molecular weight of benzoic acid.

Calculate the percent error
Figure 1
MATERIALS
LabQuest2 with App utility clamp
Temperature Probe 18  150 mm test tube
400 mL beaker lauric acid
ring stand benzoic acid
1 “The Computer - Based Laboratory,” Journal of Chemical Education: Software , 1988, Vol. 1A, No. 2, p. 73.