Chemistry lab

DETERMINATION OF MOLAR MASS BY FREEZING-POINT DEPRESSION | 141

Determination of Molar Mass by Freezing-Point Depression OBJECTIVES:

• Gain familiarity with colligative properties of nonelectrolyte solutions • Find the molar mass of a solute by the method of freezing-point depression • Evaluate the accuracy of the method by comparison to a molar mass calculated from a

molecular formula. DISCUSSION: When a solution forms, the freezing point of the solution is lower than the freezing point of the pure solvent. The magnitude of this freezing-point depression (∆Tf) depends only on the number ratio of solvent and solute molecules (or ions) in the solution, not on their chemical identity. This makes ∆Tf one of the colligative properties of solutions.

If we express the solution concentration as a molality defined so that 1 m = 1 mol solute 1 kg solvent

⎛

⎝ ⎜

⎞

⎠ ⎟ , then

the freezing-point depression follows a simple direct proportionality relationship: ∆Tf = Kf·m (1) where the proportionality constant, Kf, is called the molal freezing-point depression constant. Lauric acid (the solvent in this experiment) has a reported Kf = 3.9 °C·kg/mol = 3.9 °C/m. In this experiment, you will determine the freezing point of the pure solvent, CH3(CH2)10COOH (lauric acid). You will then find the freezing point of a lauric acid solution that contains a measured mass of solute C6H5COOH (benzoic acid) and determine the freezing-point depression. Using the experimental ∆Tf value and the reported Kf value in equation (1) will enable you to find the amount (in moles) of benzoic acid in your solution. This value, along with the known mass of benzoic acid, leads to the molar mass determination. PROCEDURE: Part I–Freezing point of pure lauric acid 1. Set up two water baths (one at about 80 °C and one at room temperature) using 400-mL

beakers filled to about 300 mL. Obtain a sample of pure lauric acid in a sealed test tube. Remove the stopper with care and then clamp this tube in a hot water bath to melt the solid lauric acid.

2. Once the lauric acid has melted completely, insert a thermometer or temperature probe into the hot liquid lauric acid. Wait for the temperature reading to achieve a steady value above 50 °C.

3. Remove the test tube/thermometer assembly from the hot bath and clamp it suspended in the room temperature bath. Insure that the water level outside the test tube is higher than the lauric acid level inside.

4. Read and record the temperature every 20 s for ten minutes. If the initial temperature is below 50 °C, begin again.

5. Continuously stir the lauric acid during cooling, using a slight up-down motion of the thermometer. When the lauric acid freezes solid, stop stirring so as not to break the thermometer or the test tube.

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6. Re-melt the lauric acid sample in the hot bath to free the thermometer. Carefully wipe any excess liquid from the probe with a paper towel, reseal the lauric acid test tube, and return it to the rack.

7. Prepare a graph of the pure lauric acid temperature vs. time. This type of graph is called a cooling curve. The freezing point of lauric acid should be apparent from the graph. Check your result with your instructor.

Part II–Freezing point of lauric acid solution of benzoic acid 8. Obtain a sealed test tube containing a mixture of benzoic acid in lauric acid. Record the

masses of benzoic acid and lauric acid exactly as they appear on the label of your tube. 9. Repeat steps 2-7 to establish the cooling curve of this mixture. Make a record of the

temperature at which solid crystals first appear in the test tube. 10. To determine the freezing point temperature of your benzoic acid-lauric acid solution, you

will need to find the temperature at which the mixture first started to freeze. Recognize that as one component freezes out of the liquid phase, the solution composition changes, which then lowers the freezing point of the remaining solution–a continual process.

DATA ANALYSIS: 1. Calculate the freezing-point depression of the benzoic/lauric acid solution. Remember that

∆Tf = Tf, pure – Tf, solution. 2. The value of the molal freezing-point depression constant of lauric acid has been determined:

K f = 3.9 °C · kg lauric acid

mol solute . Use this fact and the relationship ∆Tf = Kf · m (where m is the

molal concentration) to calculate the experimental molal concentration (also called molality) of your benzoic/lauric acid solution in mol/kg units.

3. Calculate the amount (in mol units) of benzoic acid solute present in your solution, using the mass of lauric acid recorded from the mixture label and your result from step 2 above.

4. Calculate your experimental value of the molar mass of benzoic acid, using the mass of benzoic acid recorded from the mixture label and your result from step 3 above.

5. Calculate the expected value of the benzoic acid molar mass from its formula: C6H5COOH. 6. Calculate a percent error value to compare your experimental molar mass to the expected

molar mass of benzoic acid.

DETERMINATION OF MOLAR MASS BY FREEZING-POINT DEPRESSION | 143

Name Date Score Prelaboratory Assignment

For full credit, show the detailed steps of each calculation below. Use more paper if needed. 1. The freezing point of diet soda is higher than the freezing point of regular soda, but lower

than 0 °C, the freezing point of pure water. a) Explain why both diet and regular soda freeze at temperatures lower than 0 °C.

b) Why does diet soda freeze at a higher temperature that regular soda?

2. In order to find the molar mass of an unknown compound, a research scientist prepared a solution of 0.930 g of unknown in 125 g of a solvent. The pure solvent had a freezing point of 74.2 °C, and the solution had a freezing point of 73.4 °C. Given the solvent’s freezing- point depression constant, Kf = 5.50 °C/m, find the molar mass of the unknown.

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3. The freezing point of a 1 molal aqueous solution of the nonelectrolyte ethylene glycol (the principal constituent of automotive antifreeze) is about –2 °C. The freezing point of a 1 molal aqueous solution of NaCl, a strong electrolyte, is about –4 °C. If freezing-point depression is a colligative property, it should depend only on the concentration of dissolved particles. Explain this apparent discrepancy.

DETERMINATION OF MOLAR MASS BY FREEZING-POINT DEPRESSION | 145

Name Partner Score Data Table 1. Pure lauric acid

time (s)

temperature (°C)

time (s)

temperature (°C)

time (s)

temperature (°C)

0 220 440

20 240 460

40 260 480

60 280 500

80 300 520

100 320 540

120 340 560

140 360 580

160 380 600

180 400 620

200 420 640

lauric acid in mixture g benzoic acid g Data Table 2. Benzoic/lauric acid mixture

time (s)

temperature (°C)

time (s)

temperature (°C)

time (s)

temperature (°C)

0 220 440

20 240 460

40 260 480

60 280 500

80 300 520

100 320 540

120 340 560

140 360 580

160 380 600

180 400 620

200 420 640

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DETERMINATION OF MOLAR MASS BY FREEZING-POINT DEPRESSION | 147

Calculations Freezing-point depression of the benzoic/lauric acid solution:

Molal concentration (also called molality) of benzoic/lauric acid solution:

Amount of benzoic acid solute:

Molar mass of benzoic acid (experimental value):

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Molar mass of benzoic acid (expected value):

Molar mass percent error:

DETERMINATION OF MOLAR MASS BY FREEZING-POINT DEPRESSION | 149

Name Date Score Postlaboratory Assignment

For full credit, show the detailed steps of each calculation below. Use more paper if needed. 1. The freezing point of a solvent is 87.0 °C. When a 0.163-g sample of C7H6O2 was added to

4.18 g of the solvent, the solution had a freezing point of 82.5 °C. Find Kf for the solvent.

2. Given a compound’s molar mass (99 g/mol) and elemental percent composition (48.5% C, 5.05% H, 14.1% N, and 32.3% O by mass), determine the compound’s molecular formula.

3. Draw a Lewis structure of the compound described in question 2. In the correct structure, all atoms obey the octet rule and have zero formal charge.

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