Experiment 4: Molar Mass by Freezing-Point Depression

For a quick review on reactions, solutions, and Colligative Properties, refer to the following concepts:

• General Aspects of Chemical Reactions and Equations for AP Chemistry
• General Properties of Aqueous Solutions for AP Chemistry
• Precipitation Reactions for AP Chemistry
• Oxidation–Reduction Reactions for AP Chemistry
• Coordination Compounds for AP Chemistry
• Acid–Base Reactions for AP Chemistry
• Concentration Units for AP Chemistry
• Electrolytes and Nonelectrolytes for AP Chemistry
• Colligative Properties for AP Chemistry
• Colloids for AP Chemistry

Synopsis

The molar mass (molecular mass) of a solute is determined by measuring its effect on the freezing point of a solvent. A cooling curve is constructed by plotting the temperature of a solution that is slowly cooling versus time. After the solution completely freezes, the difference between the solution's freezing point and the pure solvent's freezing point is calculated. The change in the freezing point is then related to the molality of the solution.

Equipment

beaker(s)
ice
pipet
stirrer
stopwatch
test tube(s)
thermometer
support stand
clamp
wire gauze

Measurements

1. the mass of the empty test tube
2. the mass of the test tube plus the solvent
3. the mass of the test tube plus solute plus solvent

The solid solvent or solution is heated above its melting point and then allowed to cool.

4. repeated measurements of the temperature
5. the times at which the preceding measurements were made

The above series of measurements is done one time for the solvent and one time for the solution, or the melting point of the solvent may be obtained (not measured) from a table.

Calculations

The mass of the solvent is calculated from the difference between measurements 1 and 2. The mass of the solute is calculated from the differences between masses 2 and 3. The mass of the solvent is converted to kilograms.

The temperature and time measurements (4 and 5) are plotted, and a smooth curve is drawn.

The temperature difference between the "level" regions of the solvent plot and the solution plot (or the difference between the solution plot and the tabulated freezing point of the solvent) is used to calculate the change in temperature (ΔT ) between the freezing point of the solvent and the solution. The change in temperature divided by the freezing-point depression constant (from a table) will give the molality of the solution. The molality of the solution times the kilograms of solvent yields the moles of solute. Finally, the mass of the solute divided by the moles of solute gives its molar mass.

Comments

One variation in this experiment is to add the solute to the test tube before the solvent.