Using Freezing-Point Depression to Find Molecular Weight
Recommended for 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
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 features the following Vernier sensors and equipment.