Calculation of Molecular Weight from Freezing Point Depression Data


The freezing point of a solution that contains 1.00 g of an unknown compound, (A), dissolved in 10.0 g of benzene is found to be 2.07 oC. The freezing point of pure benzene is 5.48 oC. The molal freezing point depression constant of benzene is 5.12 oC/molal. What is the molecular weight of the unknown compound?


  1. Step 1: Calculate the freezing point depression of the solution.

    deltaTf = (Freezing point of pure solvent) - (Freezing point of solution)
    (5.48 oC) - (2.07 oC) = 3.41 oC

  2. Step 2 : Calculate the molal concentration of the solution using the freezing point depression.

    deltaTf = (Kf) (m)

    m = (3.41 oC) / (5.12 oC/molal)

    m = 0.666 molal

  3. Step 3: Calculate the molecular weight of the unknown using the molal concentration.


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