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Using Freezing-Point Depression to Find Molecular Weight

Figure from experiment 4 from Advanced Chemistry with Vernier


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

\Delta t = {K_f} \times m

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).


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.

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Standards Correlations

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Experiment 4 from Advanced Chemistry with Vernier Lab Book

<i>Advanced Chemistry with Vernier</i> book cover

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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.

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