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Online Chem Labs | Learning Objectives

Background:

This experiment allows students to determine the molecular weight of an unknown organic compoundusing freezing point depression. A measured mass of an unknown is dissolved in a measured amount of benzophenone. A freezing point calorimeter is used for the measurements. The samples are placed in the calorimeter, melted, and allowed to cool slowly. A cooling curve of temperature versus time is observed, and the freezing point depression of the mixture is measured. Software is used to extrapolate the horizontal plateau back to the cooling curve of the melt, avoiding complications caused by supercooling of the melt as freezing is delayed. Using the known freezing point depression constant of benzophenone, the solution molality is calculated and the molecular weight of the unknown is then determined.

 Freezing Point Depression

Learning Objectives:

  1. Become familiar with the operation of a freezing point calorimeter.
  2. Learn how to interpret cooling curves complicated by curvature due to Newton's Law of cooling and the supercooling which frequently interferes with the onset of freezing.
  3. Use the equation that describes freezing point depression given the value of the depression constant of benzophenone to calculate the molality of the contaminated benzophenone. 
  4. Be able to calculate the approximate molecular weight once the molality is known.