Cryoscopic Determination of Molecular Weight Pre-Lab Questions (10 Points) Lab D

Question

Cryoscopic Determination of Molecular Weight Pre-Lab Questions (10 Points) Lab Day Name Lab Instructor Preliminary Questions (Show calculations on the back of the page.) 1. When a solid is melted, we usually get a curve similar to that shown in Figure 1 Date (page132). Why do we get a flat portion in the melting curve? A solution is made by mixing 1.08 g of an unknown non-volatile non-electrolyte with 10.00 g of benzene. The freezing point of pure benzene is 5.5 C. The molal freezing point depression constant, Ks, for benzene is 5.12°C/molal. The following data were obtained in a freezing point depression experiment. 2. Time, min 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 Temperature 23.8 22.1 20.2 18.5 16.6 14.8 12.7 10.8 9.1 7.2 5.5 3.6 Temperature C 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 25.0 0.5 0.1 0.8 0.9 0.8 0.7 0.6 0.5 0.4 0.2 -0.5 1.2 -2.2 A. Plot a graph of time/temperature, using the y-axis for temperature and the x-axis for time. Determine the freezing point of the solution. 141

Explanation / Answer

1)

The flat portions of each curve specify the change of phase of the substance in a heating curve

The substance is heated usually when it is in the solid phase until it becomes a gas at the end. Heating involves addition of heat energy. When heat is added, it increases the average kinetic energy of the molecules of that substance making its temperature rise.

When the temperature reaches the melting point of the substance, heating it more does not increase the average kinetic energy of the molecules, but increases the potential energy of those molecules.

During melting process, both stages, solid and liquid, exist at the same place at the same time at the same temperature.

For eg , at melting point (0°C), ice and liquid water molecules have the same "average kinetic energy" (temperature), but liquid water molecules at 0°C have more "potential energy" (money in the bank) than ice molecules.

When all the solid has melted, addition of heat increases the kinetic energy of the liquid molecules, making the temperature rise up to its boiling point, where another phase transition occurs rapidly without a change in temperature.

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