Using Freezing-Point Depression to Find Molecular Weight
Recommended for grades 9–12.
Introduction
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
where Δt is the freezing point depression, Kf is the freezing point depression constant for a particular solvent (3.9°C•kg/mol for lauric acid in this experiment), 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.
Sensors and Equipment
This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):
Additional Requirements
You may also need an interface and software for data collection. What do I need for data collection?
Download Experiment Preview
The student-version preview includes:
- Step-by-step instructions for computer-based data collection
- List of materials and equipment
Note: The experiment preview of the computer edition does not include essential teacher information, safety tips, or sample data. Instructions for Logger Pro and other software (such as LabQuest App or TI handheld software, where available) are on the CD that accompanies the book. We strongly recommend that you purchase the book before performing experiments.
Standards Correlations
See all standards correlations for Advanced Chemistry with Vernier »
Included in the Lab Book
Vernier lab books include a CD with word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.