Lab 2.docx - Using Freezing-Point DepressionComputer When a...

Computer
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).
In this experiment, you will first find the freezing temperature of the pure solvent, lauric acid,
CH
3
(CH
2
)
10
COOH. You will then add a known mass of benzoic acid solute, C
6
H
5
COOH, to a known
mass of lauric acid, and determine the lowering of the freezing temperature of the solution. In an
earlier experiment, you observed the effect on the cooling behavior at the freezing point of adding a
solute to a pure substance. By measuring the freezing point depression,  T , and the mass of benzoic
acid, you can use the formula above to find the molecular weight of the benzoic acid solute, in
g/mol.
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.
 Examine the freezing curves for each.
 Calculate the experimental molecular weight of benzoic acid.
 Compare it to the accepted molecular weight for benzoic acid.
Figure 1
MATERIALS
computer utility clamp
Vernier computer interface 18

150 mm test tube
Logger Pro lauric acid
Temperature Probe benzoic acid
400 mL beaker thermometer
ring stand
PROCEDURE

Computer 15
1. Obtain and wear goggles.
2. Connect the Temperature Probe to the computer interface. Prepare the computer for data
collection by opening the file “15 Freezing Pt Depression” from the Chemistry with Vernier
folder.