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Molar Mass by Freezing Point Depression Essay


Weigh the test tube. Assemble the apparatus of thermometer, split stopper, and stirrer. Measure about eight grams of BHT into the test tube. Record the combined mass, and the mass of the BHT. Clamp the test tube in the beaker and insert the thermometer and stirring wire assembly into the test tube and clamp the assembly to the ring stand. The thermometer should on top of the solid. Add water to the beaker so that the solid is well below the level of the water. Heat the water bath to about 90° Celsius. Allow the BHT in the test tube to melt. When the temperature of the BHT is 80° Celsius or higher, remove the thermometer and test tube from the water bath.

The thermometer should maintain position in proportion to the BHT, but the test tube should be out of the water bath beaker. Record the temperature of the BHT every 20 seconds for 30 minutes. Stir the BHT until it solidifies. Measure and record approximately one gram of cetyl alcohol. Place the cetyl alcohol in the test tube containing the BHT. Replace the test tube to the water bath. Heat the test tube until the substances are melted, and reach a temperature over 80° Celsius. Once it exceeds 80° C, remove the test tube from the water bath but maintain the thermometer position in proportion to the mixture. Record the temperature of the BHT every 20 seconds for 30 minutes. Stir the mixture until it solidifies. Repeat the above steps with fresh BHT and, a clean test tube, thermometer, stirrer, and approximately one gram of the unknown compound in place of the cetyl alcohol.


For the lab, the calculated percent error was 2.99%. The percent error came from the difference in the molar mass of the unknown substance as experienced in the lab, and in the accepted data. A possible point of error was in whether or not a group stirred the substance as it was cooling. This would affect the data recorded, and skew the results depending on how constant the stirring was between trials. If the lab were to be preformed again, the students would all have the same amount of stirring of the cooling substance, either none at all, or a constant stirring throughout the 30 minute cooling period.


The purpose of this lab was to determine the molar mass of an unknown substance by measuring the freezing point depression of a solution of an unknown substance and BHT, and to compare the data collected with the accepted value for the lab. The purpose of the lab was fulfilled as the students determined the molar mass of the unknown substance, despite much struggle throughout the process of the lab. Much of the class data was extremely off of the range of acceptable margin of error, so all groups are using the data from a successful group to perform the report.

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