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Experiment 3

# Experiment 3 - EXPERIMENT 3 MW by Freezing Point Depression...

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EXPERIMENT 3 MW by Freezing Point Depression 21 E XPERIMENT 3 Molecular Weight Determination by Freezing Point Depression 3.1 Purpose In experiment 3 , the freezing point depression constant for water will be evaluated and used to determine the molecular weight of a given substance. 3.2 Background The freezing point and the boiling point of a solvent are affected by the addition of a solute. The amount by which the freezing point is depressed or the boiling point is elevated depends on the nature of the solvent, the amount of solvent, and the number of particles of solute, but it does not depend on the nature of the solute. Since the magnitude of the boiling point elevation and freezing point depression is the same regardless of the identity of the solute, boiling point elevation and freezing point depression are called colligative properties . Suppose that the freezing point of a pure substance is some temperature FP T . At the freezing point, an equilibrium exists between the solid state and the liquid state. Thus, the overall rate at which liquid freezes must equal the rate at which solid melts. If one rate were faster than the other then one of two things would happen: i) the solid would melt as more molecules went into the liquid, shifting the equilibrium irreversibly to the liquid state, or ii) the liquid would freeze as more molecules aggregate to the surface of the solid, shifting the equilibrium irreversibly to the solid state. This equilibrium is illustrated in Figure 3-1. At any given time, a certain number of molecules N S L will break away from the solid compound and migrate into the surrounding liquid phase. At the same time, a certain number of molecules N L S that are part of the liquid collective surrounding the solid, and that are in close vicinity to the solid compound, will aggregate with the solid. At temperature FP T , the numbers N S L and N L S are equal. The processes described here represent what is commonly understood as melting and freezing , respectively.

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MW by Freezing Point Depression 22 Figure 3-1 : Melting/Freezing Equilibrium at FP T Figure 3-2 shows a typical cooling curve of a liquid. In a cooling curve, the change in temperature of the system is monitored over time. Initially, the temperature of the liquid constantly decreases. The horizontal line indicates the measured freezing point of the liquid. At this point, solid and liquid coexist. As one continues to remove heat from the system, more and more liquid freezes. Once all liquid has frozen, the temperature of the solid begins to drop. Figure 3-2 : Cooling curve of a pure liquid Also shown in Figure 3-2 is the effect of supercooling , which is the process of chilling a liquid below its freezing point. The temperature of the liquid falls below the actual freezing point, before crystals begin to form. Once the liquid begins to crystallize, the temperature rises abruptly due to the energy released during crystallization. To prevent
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Experiment 3 - EXPERIMENT 3 MW by Freezing Point Depression...

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