MHS Chemistry
Freezing Point Depression & Molecular Weight

The purpose of this activity is to determine the molecular weight of a known substance by measuring the freezing point depression produced in a known solvent. The calculated molecular weight will be compared to the "official" value.


  1. The solvent is p-dichlorobenzene (also known as 1,4-dichlorobenzene). The freezing point depression constant for this substance is k = 7.10 C°/m. The solute can be either naphthalene (C10H8) or iodine (I2). You get to choose.
  2. Calculate the molecular weight of your chosen solute.
  3. Measure the mass and melting point of p-dichlorobenzene in a medium test tube.
  4. Add some of the solute (no more than about 1/20th of the solvent mass). Measure the new melting point, and determine the mass of solute added.
  5. Use the formula DT = km and the definition of molality to calculate the moles, then molecular weight of the solute.
  6. If your answer in #2 is considered "correct," what is your percent error in #5?


[Freezing Point Depression & Molecular Weight score sheet][MHS Chemistry page]