04 Freeze Pt LQ (Autosaved).docx - LabQuest Using...

LabQuest
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

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 the pure solvent, lauric acid.
 Determine the freezing temperature of a mixture of lauric acid and benzoic acid.
 Calculate the freezing point depression of the mixture.
 Calculate the molecular weight of benzoic acid.
m
Figure 1
MATERIALS
LabQuest lauric acid, CH
3
(CH
2
)
10
COOH
LabQuest App lauric acid-benzoic acid mixture
Temperature Probe hot water bath
two 18  150 mm test tubes utility clamp
400 mL beaker paper towel or tissue
ring stand