LABORATORY ACTIVITY:
                                   STUDENT VERSION

Activity 2: The Freezing Point Depression of Lauric Acid

        Introduction
                Dissolved solute particles disrupt the crystallization process and lower the freezing point of the solvent. The amount the freezing
                point is depressed is directly proportional to the number of moles of particles dissolved.

        Purpose
                To determine the freezing point of lauric acid and to observe the effect a solute has on the freezing point.

        Safety
                1. Wear protective goggles throughout the laboratory activity.
                2. Proper laboratory safety procedures must be followed.
                3. The burners heat metal and glass, so proper care must be taken not to burn yourself. Hot and cool glass look alike.
                4. Any substances spilled on you should be immediately washed off with large amounts of water and the incident reported
                    to your teacher.

        Procedure

    Part I: The Freezing Point of Lauric Acid
        1. Prepare one hot water bath (65 °C) and one warm water bath (30 °C). Set up two ring stands and
            place 400 mL water into each 600-mL beaker; using the burner, heat one bath to 65 °C and warm the
            other bath to 30 °C.
        2. Weigh the test-tube (25 x 200-mm) empty; fill the test-tube to a height of about 5 cm with solid lauric
            acid and weigh the test-tube and lauric acid.
        3. Clamp the test-tube with the lauric acid in the hot water bath and allow the acid to melt.
        4. When the lauric acid has just melted, remove the test-tube from the water bath and place the
            thermometer and stirrer into the lauric acid.
        5. Place the test-tube containing the melted lauric acid, thermometer and stirrer into the 30 °C water
            bath and clamp the test-tube in place. Stir the lauric acid and record the temperature every 30 seconds
            until the temperature of the lauric acid has dropped to 40 °C. CAUTION: Do not attempt to remove or
            even move the thermometer or stirrer from the solidified lauric acid!

        Part II: The Freezing Point Depression of Lauric Acid
         6. Weigh 2.5 g benzoic acid.
         7. Remelt the lauric acid as you did in Step 3 above, and as soon as the lauric acid is melted, dissolve the
             benzoic acid in the lauric acid. Take care when adding the benzoic acid to make sure that all of it
             adds to the lauric acid, and that none of it sticks to the sides of the test-tube. You will need to stir the
             mixture vigorously with your stirrer in order to get the benzoic acid to dissolve completely.
          8. Place the test-tube containing the melted lauric acid, benzoic acid, thermometer and stirrer into the 30
              °C water bath and clamp the test-tube in place. Stir the mixture and record the temperature every 30
              seconds until the temperature of the mixture has dropped to 36 °C.
          9. Melt the mixture in the hot water bath and carefully remove the thermometer and stirrer from the
              mixture. Quickly wipe the thermometer and stirrer with a few layers of paper towels before the mixture
              has had a chance to solidify onto the thermometer.
        10. Dispose of the paper towels and the test-tube contents as directed by your teacher, and return all
              materials to their proper places.
        11. Thoroughly wash your hands before leaving the laboratory.
 

        Data Analysis
            1. Graph the temperature vs. time for each trial.
            2. What is the freezing point of lauric acid? Explain your reasoning.
            3. What is the freezing point of the mixture of lauric acid and benzoic acid?  Explain your reasoning.
            4. How many moles of benzoic acid were used?
            5. Calculate the molality (mol of benzoic acid/kg of lauric acid) of the solution.
            6. Calculate the change in the freezing point temperature in going from the pure solvent to the solution.
            7. Calculate the freezing point depression constant (K f ) and compare this with class results. Kf  =
                change in freezing point divided by molality of solution. Remember that molality is moles of solute per
                1000 g of solvent.
 

        Implications and Applications
            1. Is the freezing point the same as the melting point for pure lauric acid? Why?
            2. Explain what is happening to the energy removed from the lauric acid in each part of the graph for the
                cooling of the pure lauric acid.
            3. What is happening, on the molecular level, during the time when the graph of the pure lauric acid is a
                flat, straight line?
            4. Why is the graph of the pure lauric acid different from the graph of the mixture of acids?
            5. What is happening, on the molecular level, during the time when the graph of the lauric acid plus
                benzoic acid shows a minimal temperature change with time?

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