CHM 152 updated May, 2011 Lab 3: Freezing-Point Depression

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1 CHM 152 updated May, 2011 Lab 3: Freezing-Point Depression Introduction This lab will examine the effect a solute has on a liquid s freezing point and show how freezing-point depression can be used to determine the molar mass of an unknown solute. In Part I you will experimentally determine the freezing point of naphthalene. Since this compound is already a solid a room temperature, you will melt it and allow it to refreeze. In Part II you will experimentally determine the freezing-point depression constant (K f ) of naphthalene by preparing a solution using a known solute, biphenyl (C 12 H 10 ). Using the change in freezing point temperature (ΔT f ) and the molality of our solution (m), we can calculate the K f of naphthalene using the equation for freezing-point depression ΔT f = K f m In Part III you prepare a similar solution, only this time the identity of the solute is not known. You will attempt to identity the solution by first determining the molality of the solution. As with Part II, you start with the equation for freezing-point depression, but here you use the ΔT f of your unknown solution the K f of naphthalene to calculate m. Once you have the molality of the solution, you can calculate the solute s molar mass. In each case you ll use a plot of temperature over time to determine the freezing point of your sample, similar to those shown in Figures 1 and 2. The dip that you see is due to an effect known as supercooling that often occurs during the freezing process. You may or may not actually observe this, as it could occur between data readings. 87 liquid temperature, C freezing point solid time, min Figure 1: Freezing point of naphthalene

2 temperature, C liquid solid freezing point time, min Figure 2: Freezing point of a naphthalene solution Concepts to Review Freezing-point depression Molality

3 Procedure Part I: The Freezing Point of Naphthalene 1. Assemble a hot bath as follows: a) Fasten a support ring to a ring stand b) Set a wire gauge on the support ring c) Place a mL beaker of water on the wire gauge. Fasten a second support ring around the beaker to reduce the risk of knocking it over. d) Attach a clamp to the ring stand just above your beaker. This will be used to hold your sample (step 4). e) Add a couple of boiling chips to the water to reduce the risk of uneven boiling ( bumping ) 2. Use a Bunsen burner to pre-heat the water as you prepare your sample. 3. Weigh out about 6g of naphthalene and transfer it to a large test tube. Cap the test tube with a rubber stopper that s threaded with a thermometer and wire stirrer (this item will be pre-assembled for you; see instructor for its location). The stopper doesn t have to fit in the opening of the test tube. It can simply be set on top of it. 4. Set the apparatus in the beaker of hot water. Use the clamp from step 1d to keep the sample submerged in the water without touching the bottom of the beaker. 5. Heat your sample, stirring occasionally with the wire stirrer, until the sample temperature reaches C. The sample should be completely melted by this point. 6. Turn off the burner and allow the sample to cool (leave the apparatus in the bath), stirring the sample as it cools. Record the temperature of your sample in one-minute intervals. When you first begin to see crystals form, mark the temperature with an asterisk (*). 7. Continue cooling until the entire sample has solidified. 8. Prepare a plot of temperature over time and use this to determine the freezing point of pure naphthalene. For simplicity, use an XY scatter plot connect by smoothed lines. Format the temperature axis to 1º increments. Part II: Determining the K f of naphthalene 1. Reheat the naphthalene from Part I to a temperature of C. 2. Add approximately 1g of biphenyl to your sample of naphthalene and stir. 3. Repeat steps 6-8 from Part I. 4. See the Waste Disposal section for instructions on how to discard your used solution. Any traces of solid left in the test tube or on your thermometer can be removed with acetone. Rinse the test tube with water and dry thoroughly using a cool (yellow) flame from your Bunsen burner. Part III: Determining the molar mass of an unknown solute Repeat Part II using a fresh sample of naphthalene and substituting biphenyl with your assigned unknown. Waste Disposal Reheat your solutions until it has completely melted, then dispose of it in the organic waste bottle (not water soluble). If the solution refreezes as your pour it into the bottle, simply melt it again with a cool flame. Trace amounts of solid can be removed with acetone or simply scraped off with a spatula.

4 Name: Section: Data Part I: The Freezing Point of Naphthalene 1) Mass of naphthalene (g) 2) Temperature of sample over time. time (min) temperature ( C) time (min) temperature ( C) 3) Prepare a plot of temperature vs. time for this sample. Print and include in your report. 4) Freezing point of naphthalene ( C)

5 Part II: Determining the K f of naphthalene 1) Mass of biphenyl (g) 2) Moles of biphenyl 3) Molality of solution (m) Show your work for calculating the molality of your solution 4) Temperature of sample over time. time (min) temperature ( C) time (min) temperature ( C) 5) Prepare a plot of temperature vs. time for this solution. Print and include in your report. 6) Freezing point of solution ( C)

6 7) Freezing-point depression, ΔT f ( C) 8) Freezing-point depression constant, K f ( C/m) Show your work for calculating K f Part III: Determining the molar mass of an unknown solute Unknown code 1) Mass of solute (g) 2) Mass of naphthalene (g) 3) Temperature of sample over time. time (min) temperature ( C) time (min) temperature ( C) 4) Prepare a plot of temperature vs. time for this solution. Print and include in your report.

7 5) Freezing-point of solution (ºC). 6) Freezing-point depression, ΔT f ( C) 7) Molality of solution (m) Show your work for calculating the molality of this solution. 8) Moles of solute in solution Show your work for calculating the moles of solute 9) Molar mass of unknown Show your work for calculating the unknown s molar mass

8 Name: Section: Post-Lab Questions 1. What effect would each of the following have on your calculated value of K f (too high, too low, or no effect)? In each case explain your answer. a) There was a small bubble in the thermometer, causing all the temperature readings to be erroneously high. b) A student used a new thermometer for Part II, but failed to notice that some of the naphthalene from Part I was sticking to the old thermometer. c) After melting the naphthalene, small pieces of rubber from the stopper are seen floating in the solution. 2. A student decided to use the same weighing boat he used to measure the biphenyl in Part II for measuring his unknown in Part III. However, he failed to notice that a small amount of biphenyl was still in the boat, which was transferred to the test tube with his 1g of unknown. What effect would this have on each of the following (too high, low, or no effect)? In each case, explain your answer. a) The measured ΔT f b) The calculated molality of the solution c) The calculated molar mass of the unknown

9 Name: Section: Pre-Lab Questions 1. Define the following a) Molality b) Colligate property 2. Pure nitrobenzene freezes at 5.67 C. When 1.0g of ethanol (C 2 H 6 O) is mixed with 20.0g nitrobenzene, the freeze point drops to 0.53 C. What is the freezing-point depression constant (K f ) of nitrobenzene? 3. Cyclohexane has a freezing point of 6.6 C and a K f of 20.0 C/m. 1.50g of an unknown solute is dissolved with 18.0g of cyclohexane. The freezing point of the solution was 1.3 C. Using this information, and assuming the solute doesn t dissociate, calculate the following a) The molality of this solution b) Moles of solute present c) Molar mass of the solute