Question

How to calculate mass of unknown solid from its freezing point

Answer 1

Lab 7 - Determination of the Molar Mass of an Unknown Solid by Freezing Point DepressionGOAL AND OVERVIEWIn the first part of the lab, a series of solutions will be made in order to determine the freezing point depression constant, Kf, for cyclohexane. The freezing points of these solutions, which will contain known amounts of p-dichlorobenzene dissolved in cyclohexane, will be measured. In the second part of the lab, the freezing point of a cyclohexane solution will be prepared that contains a known mass of an unknownorganic solid. The measured freezing point change will be used to calculate the molar mass of the unknown solid.Objectives of the Data Analysis

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Use the mathematical relationship between freezing point depression and solution molality.SUGGESTED REVIEW AND EXTERNAL READING

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Reference information on thermodynamics; data analysis; relevant textbook informationBACKGROUNDIf a liquid is at a higher temperature than its surroundings, the liquid's temperature will fall as it gives up heat to the surroundings. When the liquid's temperature reaches its freezing point, the liquid's temperature will not fall as it continues to give up heat to the surroundings. Instead, the liquid will begin to freeze. Those parts closest to the cold surroundings will turn from liquid to solid.This freezing process releases a quantity of heat energy that is equal in magnitude but opposite in sign to the heat of fusion, ΔHfusion, of the substance. ΔHfusion is defined as the heat absorbed by a solid in the process of melting. When the liquid freezes to form the solid phase, it must give up the same amount of heat: ΔHfreezing = – ΔHfusion.The temperature of the system (the freezing liquid) remains at the freezing point until all of the liquid has frozen. If you plot temperature against time for this process, the curve will resemble that shown. Once the liquid has completely frozen, only then will the temperature start to fall again.