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Using Freezing-Point Depression to Find Molecular Weight

Experiment #4 from Advanced Chemistry with Vernier

Education Level
High School


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

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This experiment is #4 of Advanced Chemistry with Vernier. The experiment in the book includes student instructions as well as instructor information for set up, helpful hints, and sample graphs and data.

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