1516L Experiment 3

COLLIGATIVE PROPERTIES

Objectives

The primary objective of this experiment is to determine the molecular weight of an unknown from its freezing point depression in tertiary-butanol, or t-butanol for short.

Discussion

Colligative properties are useful for determining molecular weights of unknown samples, because they depend only upon the amount of sample dissolved in a known solvent, and not on the identity of the sample. Colligative properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure. In this experiment you will first determine the freezing point of pure t- butanol, which we will use as a solvent, and also the freezing points of two solutions of an unknown mixed with t-butanol. From the resulting change in temperature (T) that you observe, you will be able to calculate the molar mass of the unknown.

The equation which relates T to the concentration of the solute in a known solvent is:

T = Kf · m (1)

where Kf is the freezing point depression constant and m is concentration in molality units. The freezing point depression constant in equation (1) depends only on the solvent. The solvent we will use in this o experiment is t-butanol, for which Kf = 8.37 C/m. Recall that molality has units of moles of solute per kilogram of solvent. The unit of solution concentration that you may be more familiar with is molarity, which is moles of solute per liter of solution. The problem with using molarity in this experiment is that this quantity can change depending upon temperature. For example, as temperature increases the volume of a solution usually increases meaning that the molarity decreases. Since this experiment is based on observed freezing point changes for a pure solvent vs. a solution, we must use a unit of concentration that does not change with temperature.

At first glance you may wonder how you will be able to use equation (1) to determine the molar mass of your unknown. This should become clear if we rewrite equation (1) as follows:

T = Kfm = Kf · moles solute = Kf · grams solute/molar mass of solute (2) kg of solvent kg of solvent

The only unknown in equation (2) is the molar mass of the solute, which you can solve for algebraically.

Safety Precautions

Be sure to wear lab coats and goggles at all times during this experiment. Use labeled waste containers for your solutions when finished – do not pour anything down the sink!

Peter Norris, YSU Procedure

You will work in pairs for this exercise. The experiment involves using the MicroLab interface to obtain a total of three freezing points: one for the pure solvent, and one each for each of two unknown solutions (i.e. containing the same unknown in the same solvent, but having different concentrations). Each of the freezing points will be extracted from a cooling curve, which is a temperature vs. time plot that will be generated in real time during the experiment. The difference in freezing temperatures between the unknown solution and pure solvent is T in equation (1).

Thermocouple Calibration

Acquire a thermocouple from your instructor and then follow the instructions given in the Thermocouple Calibration document to calibrate the thermocouple using boiling water and then ice-water. Once you are back to the window below you are ready to collect data in the solubility experiment. You should have Thermocouple and Time 1 under the Sensor list; Thermocouple and Time 1 on the Y and X axes of the graph respectively, and also atop Columns A and B on the data table.

Sample Preparation

1. Obtain a large (200 mm) test tube and make sure it is very clean and dry. Use soap and water to clean it and then heat the tube carefully over a flame to make sure it is dry. Any water or other impurities that become mixed with your solvent will induce their own colligative effects and result in large experimental errors.

2. Place your test tube in a dry beaker and weigh the beaker plus the test tube. Record the mass on the report sheet.

3. Put about 10 mL of t-butanol in the test tube and weigh the beaker (the same one as before!) plus test tube plus solvent. Record this value on your report sheet.

Peter Norris, YSU 4. Prepare a warm water bath by heating about 100 mL of tap water in a 250 mL beaker with a Bunsun burner. The water should not boil; it only needs to get to about 50 °C, which you can measure with the thermometer provided.

5. Prepare a second water bath at room temperature by adding about 200 mL of tap water to a 400 mL beaker. This will be used to initiate the cooling process. Get some ice from the supply table in order to be ready to cool the sample further.

Data Acquisition and Analysis

1. Put a two-hole rubber stopper fitted with the calibrated thermocouple and stirring wire into your test tube. Warm the test tube to about 45-50 °C by clamping it in the warm (not boiling!) water bath. Once all of the material in the tube has completely melted, lower the test tube into the room temperature water bath (Figure 1) and clamp it securely. Then place the thermometer in the water bath. Make sure that the level of the water bath is higher than that of the liquid in the test tube.

2. Begin the data acquisition by clicking on the “Start” button on the computer screen. The temperature will decrease gradually and be graphed on the screen. Once the thermocouple temperature (on the computer screen) reaches about 30 °C, add a few chunks of ice to the water bath and stir the ice-water carefully with your glass stirring rod.

3. Once the thermometer shows the ice-water temperature to be about 20 °C, add more ice and stir the ice-water carefully with your glass stirring rod. As the bath temperature approaches 10 °C you should see solid beginning to form in the test tube. The graph on the computer screen should begin to level off as the solid freezes. Continue cooling for several more minutes until the graph temperature starts to decrease again and most of the material in the test tube has frozen. At this point halt the data acquisition by clicking on the “Stop” button. Your graph should look roughly like the one below but the numbers will probably be different.

Peter Norris, YSU 4. Click the “Analysis” button, and then “Add curve fit.” Click on the “Time” box and then enter the time limits that define the beginning of the straight portion of the initial cooling slope (see above picture) and the point where the slope begins to level off; then choose “First order” and press “OK.”. Label the analysis “Freezing 1,” then hit “OK.” You should see a line fitted to that part of the cooling curve.

5. Click the “Analysis” button again, then “Add curve fit.” Click on the “Time” box and then enter the time limits that define the beginning of the horizontal portion of the cooling curve (see above picture) and the point where the slope begins to level off again; then choose “First order” and press “OK.” Label the analysis “Freezing 2,” then hit “OK.” You should see a line fitted to that part of the cooling curve.

6. Hover the cursor over the point at which the two lines intersect and you should see a box appear with “X = …, Y = …” Record these values; you will be using the Y value as the freezing temperature of t-butanol in order to calculate T in equation (2).

7. The rest of the experiment is carried out as described in the 1516L manual, i.e. parts 4-7 of the Data Acquisition and Analysis section of the Colligative Properties exercise.

Peter Norris, YSU