Determination of Molecular Weight by Freezing Point Depression

Chem. lab 112-02

Experiment 13 Prelab: Determination of Molecular Weight by Freezing Point Depression

Summary of Principals:

Determination of the molecular weight of a substance, using colligative properties and the freezing point depression.

Prelab Questions:

  1. Define:
    1. Solution: A homogenous mixture which is uniform and consists of two or more substances in the same phase
    2. Solute: the substance being dissolved
    3. Solvent: The substance that the solute has dissolved
    4. Colligative property: physical properties of a solution that depends on the number of solute particles in solution but not on the nature of the solute properties
  2. List all 4 colligative properties
    1. Elevation of boiling point
    2. Lowering of the freezing point
    3. Vapor pressure lowering
    4. Osmotic pressure
  3. Justify why the following statement is true: To obtain accurate experimental measurements one should select a liquid with a larger value of Kf
    1. The freezing point depression is proportional to the value of KF thus for a given concentration, the larger the KF, the greater the value for DeltaTF. To obtain accurate experimental measurements a liquid with a larger value KF should be used.

Equations & Mechanisms:

  • DeltaTF = ToF – TF
  • DeltaTF = KFm
  • m = (DeltaTF) / KF

Apparatus & Supplies:

  • Thermometer
  • Ring and ring stand
  • Clamp
  • Stirring wire
  • Bunsen burner
  • 450 mL beaker
  • 18 X 150 mm test tube

Outline Experimental Procedure:

Cooling curve of pure Naphthalene

  1. Clean and flame dry a large test tube. Find a 2-hole stopper of the right size
  2. Weigh the test tube and add 10-15 g of Naphthelene
  3. Set up the freezing point apparatus
  4. Fill the 450 mL beaker ¾ full of water
  5. Heat the water to about 85OC with the Bunsen burner
  6. When most of the naphthalene has melted, the insert the stopper with a thermometer and the stirring wire to the test tube
  7. Turn off the Bunsen burner
  8. Take the temperature of the sample was every 10 seconds while stirring
  9. Record the temperature until its impossible to stir the sample

Cooling curve of Naphthalene Solution

  1. Weigh 1.0 – 1.5 g of an unknown
  2. Place the test tube with the pure naphthalene in the water bath until the sample melts
  3. Remove the stopper from the test tube and add the unknown
  4. Replace the stopper and gently stir the sample
  5. Use steps 6-9 from part one of the experiment
  6. Clean the test tube according to the procedure given

Safety Precautions:

The chemicals are toxic by ingestion and skin contact. There are containers labeled for safe disposal of Naphthalene and all of its mixtures. Extreme care is needed when using the Bunsen burner and all heated objects. Safety goggles and lab coat need to be worn at all times.


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