Molecular Weight by Freezing Point Depression

Topics: Melting point, Freezing-point depression, Concentration Pages: 2 (622 words) Published: December 30, 2012
The melting point is the temperature at which the crystal structure of a solid breaks down with increasing entropy (+ΔS). Solute particles introduced into the solvent increases the entropy, and therefore decreases the temperature of the solution in order to reach the melting or freezing point of the solution. Hence the freezing point is depressed as solute is added. This depression is the difference (ΔTf) between the freezing point of the solvent and solute, and the freezing point of the pure solvent. A property which depends only on the number of particles present, and not on chemical composition is called a colligative property. Therefore, freezing point depression is such a property.

Molality (m) is used to express the concentration of a solution in terms of moles of solute per kilogram solvent. Moles of a substance is the grams per molecular weight. The freezing point depression of a solution can be expressed as ΔTf = Kfm

where Kf is the molal freezing point depression constant of the solvent. In this lab, we will use naphthalene as the solvent where Kf = 6.850C/molal.

naphthalene (moth flakes), roll sulfur, unknown molecular substances, thermometer, hot plates, test tube clamp, large test tube, wide-stem funnel, goggles

A. Freezing Point of Pure Naphthalene
(Since ordinary laboratory thermometers are often in error, do not change thermometers during this lab.) 1. Mass a clean, dry test tube.
2. Set up the apparatus as described by your instructor.
3. Add approximately 15-20 g of naphthalene to your test tube, insert the thermometer, and using the 1000C hot water bath heat to approximately 900C to melt the sample. 4. Place the test tube in the warm water bath (approximately 500C) and while stirring gently with the thermometer allow the naphthalene to cool to its freezing point. Record this temperature to 0.10C. (Watch for evidence of supercooling.)

5. Reheat the naphthalene to remove the...
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