Last edited 09/21/04

CHEM 1112

Molecular Weight by Freezing Point Depression

Purpose:

To determine the molecular weight of an unknown organic compound by measuring the freezing point depression of an aqueous solution of the compound.

Introduction

The freezing point of a solution of a nonvolatile solute is less than that of the pure solvent. The freezing point depression or change in freezing point is given by eq. 1 and 2.

D Tf = Kfm (1)

D Tf = Tf - Tf (2)

Tf is the freezing point of the solution in C and Tf is the freezing point of the pure solvent, also in C. Kf is the freezing point depression constant. Its value depends only on the solvent. For water, Kf = -1.86C/molal, m is the molal concentration of the solute, given by equation 3.

m = moles solute (3)
        kg solvent

m = (grams solute)/MW (4)
            kg solvent

The molecular weight of the solute is MW. To determine the molecular weight of an unknown from the freezing point depression, the freezing points of the pure solvent and of the solution are measured, and D Tf calculated from eq. 2. The molality of the solution is then calculated from eq. (1). The molecular weight of the solute is obtained by rearranging eq. 4 to solve for MW.

MW = grams solute (5)
           (m)(kg solvent)

In this experiment you will determine the molecular weight of an unknown substance by measuring the freezing point depression of an aqueous solution of the unknown.

Method

The method requires the determination of the freezing points of the pure solvent, water, and of a solution of the unknown dissolved in water. The freezing point of pure water is determined by cooling a sample of water and measuring its temperature as a function of time. The resulting data are used to construct a cooling curve, shown in Figure 1.

Figure 1. Cooling curve for a solution of pure water

 

The liquid may cool to a temperature below the equilibrium freezing point. This is referred to as "supercooling" (See Figure 1). Supercooling can be minimized by continuously stirring the liquid as it cools. At the freezing point the temperature remains constant because as the liquid solidifies, heat equal to the enthalpy of fusion is released. The true freezing point corresponds to the horizontal portion of the cooling curve.

The freezing point of the solution is determined in the same way as the freezing point of the solvent. The cooling curve, however, has a different appearance and is shown in Figure 2.

Figure 2. Cooling curve for a solution.

The curve does not become horizontal at the freezing point. Rather a "kink" or discontinuity occurs at the freezing point. This discontinuity may be obscured by supercooling. If so, it will be necessary to extrapolate by drawing the line ab as shown.

The reason that the cooling curve is not horizontal at the freezing point is that as the solution freezes, the composition of the liquid changes. The solid phase which forms at the freezing point is pure solvent. Consequently, as freezing occurs, the concentration of the solution increases, and the freezing point decreases.

Experimental Procedure

Part 1. Freezing point of pure water:

The apparatus to be used is shown in Figure 3. Clean and dry the smaller test tube, and add to it approximately 20 mL of deionized water. Insert the stirrer, stopper and thermometer into the tube. Make sure that the thermometer is immersed in the water, and that the stirrer moves up and down freely.

 

mwfpdep.gif (3434 bytes)

Figure 3. Apparatus for determining the freezing point of pure water or a solution.

Prepare an ice-salt bath using generous quantities of rock salt. Place a layer of crushed ice 3-4 cm deep at the bottom of a 600 mL beaker. Sprinkle a portion of the salt on the ice. Continue to alternate the layers of ice and salt until the beaker is full. Stir the ice-salt mixture well, immerse the smaller test tube that contains the water into the ice-salt bath, and allow the water to cool to about 4C. Remove the test tube from the bath, place it inside the larger test tube, and immerse the entire test tube assembly into the ice-salt bath, as shown in Figure 3. Stir the water, measure its temperature, and continue taking temperature measurements every 30 seconds. Estimate the temperature to within 0.1C. Stir the water in the test tube continuously at a uniform rate with the wire stirrer. Continue taking temperature-time readings until 6 successive readings are equal to one another. Repeat this measurement once.

Part 2. Determining the freezing point of a solution of an unknown substance.

Use the same thermometer you used in Part 1. Obtain a sample of unknown compound. Be sure to record the number of your unknown in your notebook. Your instructor will tell you the approximate mass of unknown to use in each determination. All masses should be recorded directly in your notebook.

Dry the smaller test tube and weigh it to the nearest 0.01 g. Add 20 ml of deionized water and weigh again to the nearest 0.01 g. Weigh out 3.5 g to 4 g of the unknown. WARNING: you must stirr continuously while adding the unknown to the water or it can clump. Add the weighed unknown to the test tube.  Stir the solution until the solid compound is completely dissolved. 

Reweigh the test tube and contents to the nearest 0.01 g. Insert the stopper, thermometer, and stirrer. Make certain that the thermometer bulb is immersed in the solution.Discard the ice-salt bath used in Part 1, and prepare a fresh one.

Immerse the test tube containing the solution of unknown in the bath and allow it to cool to about 1C. Remove it from the bath, place it in the larger test tube and place the entire test tube assembly back into the bath, as shown in Figure 3. Repeat the procedure in Part 1. That is, take temperature time readings every 30 seconds. Stir the solution continuously at a uniform rate during the course of the determination. Continue taking readings until the solution solidifies. Do another determination with a second portion of your unknown.

Results

          Calculations

1. For each trial, prepare a graph of time versus temperature as shown in Figures 1 and 2. Determine the freezing point from each graph.
2. Calculate the average value of the freezing point of pure water from the results of the two trials in Part 1.
3. Calculate the freezing point depression for each of the two trials in Part 2 (eq. 2)
4. For each trial in Part 2, calculate the mass of water and the mass of unknown compound.
5. Calculate the molality of each solution in Part 2 (eq. 1).
6. Calculate the molar mass of the unknown for each solution (eq. 3).
7. Calculate the average molar mass of your unknown.

Results:

1. The time versus temperature data should be arranged in tabular form for each of the four trials.
2. Graphs of time versus temperature should be either taped or glued into the notebook. Show the freezing point on each graph.
3. Show all calculations outlined above.

Conclusion: Be sure to include the following as you write your conclusion using complete sentences in well organized paragraphs.

Report the identification number of your unknown and its average molecular weight.
What is the average deviation of your two measurements of the molecular weight?
What effect would each of the following have on the calculated molecular weight of an unknown?   Would the calculated value be higher or lower than the actual value? You must rationalize your answers.
Some of the unknown does not dissolve.
The thermometer reads 0.63C higher than it should over the whole temperature range.
The test tube is not dried on the inside before it is weighed.