Freezing-Point Depression and Molecular Weight

When a solution is slowly cooled, pure solid from the solvent will begin to form at some temperature. This temperature at which solid first begins to form is the freezing point of the solution. It is important to note the temperature at which solid first forms because, unlike the case of freezing pure solvent, the temperature of the solution continues to drop at a slower constant rate as additional solvent freezes from the solution. This slow constant temperature drop occurs because the molality of the solution (and thus the freezing point depression) increases as more solvent 

if a mole is a number of particles, and if molality is the number of moles of particles per kilogram of solvent, then freezing point depression is used to count the number of solute particles per unit amount of solvent. 

In this experiment, you will measure the freezing point of pure naphthalene and the freezing point of a solution of an unknown solid 

Obtain a 25 x 200-mm test tube and a wire-loop stirrer in a slit, 4 2-hole rubber stopper from the equipment bench. Weigh your clean, 

Assemble the apparatus shown in FIGURE 20.1. Carefully place the thermometer in the slit rubber stopper. The top of the stopper should be at the 74° mark on the thermometer scale to enable you to make the necessary readings easily. 

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By referring to Experiment 20 (Freezing-Point Depression and Molecular Weight of an Unknown Solid), devise a reasonable method for determining the molecular weight of aspirin. Your instructor must approve an outline of your method and the quantities you plan to use before you begin work. Record all experimental data in TABLE 31.1C. 

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