Using Freezing Point Depression to Find Molecular Weight

3. For each unknown solution, record the mass of water, the mass of the unknown compound and calculate the molality.

(a) unknown #1

(b) unknown #2

4. Calculate the molecular weight, MW, in g/mole for each unknown.

(a) unknown #1

(b) unknown #2

5. If you are told that one of the unknowns is glucose, C₆HO₆, with a molecular weight of 180.16 g/mole, which would you say it is?

6. What effect would each of the following have on the calculated molecular weight of an unknown? Would the calculated value be higher or lower than the actual value? Explain your answers.

(a) Some of the unknown does not dissolve.

(b) The thermometer reads 0.63°C higher than it should over the whole temperature range.

(c) The test tube is not dry (and has water drops inside) before the solutions are made up.

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Solution Summary

This solution explains how to use measured freezing point depressions of water samples. Masses are used to calculate molality, which is then used to calculate the molecular weight of the two unknowns. The effects of several sources of error are then accounted for quantitatively.