Freezing-Point Depression to Determine an Unknown Compound

A temperature probe interfaced to a computer is used to acquire the temperature readings in this experiment. The temperature probe has an uncertainty of ±0.1 °C.

1. Setting the Parameters in the Software

1. Set the length of the experiment to 800 s.
2. Set the sampling rate to 1 sample per second.
3. Set the upper limit for the temperature range to 40 °C and the lower limit to 0 °C.

2. Measuring the Freezing Point of Cyclohexane

1. Dispense 12.0 mL of cyclohexane from the dispensing bottle into a test tube that is clean and dry.
   Caution: Cyclohexane is a flammable solvent.
2. Wipe the temperature probe with a lint-free wipe to be sure it is dry.
3. Insert the stopper with the temperature probe and wire stirrer into the test tube.
4. Make sure the tip of the temperature probe is in the center of the liquid and not touching the sides or bottom of the test tube.
5. Fill a 600-mL beaker about one-third full of water, and add ice until the beaker is three-fourths full.
6. Start the data collection. The computer acquires a temperature reading every second.
7. Move the test tube into the ice-water bath and hold it so the level of liquid in the test tube is below the level of water in the bath.
8. Immediately begin stirring the liquid with the wire stirrer, continuously and at a constant rate.
9. Once freezing begins, as long as liquid and solid are both present, the temperature remains constant until the entire mass has solidified. Allow the computer to continue recording the temperature until the plot has leveled off at a constant temperature.
   Note that once the cyclohexane has frozen solid, the temperature starts to decrease again.
10. When a sufficient number of data points have been collected, stop the data collection.
11. Remove the test tube from the ice-water bath and let it warm up to room temperature.
12. Save the data.
13. Adjust the y-axis limits so the plot fills the page. Title the graph, and then print it.

3. Preparing a Solution of the Unknown Compound

1. Accurately weigh 0.14 g of the solid unknown material onto a piece of weighing paper.
2. Check to be sure the cyclohexane contained in the test tube has melted.
3. Remove the stopper from the test tube and carefully add the unknown solid to the cyclohexane, avoiding the loss of any compound adhering to the sides of the test tube or stopper.
4. Replace the stopper and re-weigh the paper to account for any crystals that remain on it.
5. Stir the solution in order to completely dissolve the solid. It is important that no crystals remain.
6. Make a new ice-water bath.

4. Measuring the Freezing Point of the Unknown Compound

1. Prepare the computer to collect a second set of data.
2. Start the data collection.
3. Move the test tube that contains the solution into the ice-water bath.
4. Immediately begin stirring the solution continuously and at a constant rate.
5. Collect the data for 300–500 s in order to clearly see the change in slope that occurs as the solution freezes.
6. Stop the data collection.
7. Save the data, adjust the limits of the y-axis, title the graph, and print it.
8. Do not throw any cyclohexane or unknown compound down the sink. Pour the liquid mixture into the "Laboratory Waste" jar. Rinse the test tube and temperature probe with acetone to remove the last traces of any crystals, pouring the rinses in the waste jar.