Freezing point depression PDF

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Freezing-Point Depression Supplies: Vernier LabQuest2 Temperature probe Ring stand and clamp 2 – 400mL beakers Test tube holder

2 – 18x150mm test tubes 8g octanoic acid 0.5g benzoic acid 0.5g unknown Laptop computer with Microsoft Excel or similar program

When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property, that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is:

∆ T f =k f m Where ∆ T f

is the freezing point depression, k f is the freezing point depression constant for a particular solvent (4.70°C·kg/mol for octanoic acid in this experiment), and m is molality of the solution (in mol solute/kg solvent). This relationship can be used to determine the molecular weight of an unknown solute. Objectives In this experiment, you will:  Determine the freezing temperature of the pure solvent, octanoic acid.  Determine the freezing temperature of a mixture of octanoic acid and benzoic acid.  Calculate the freezing point depression of the mixture.  Calculate the molecular weight of benzoic acid based on the freezing point depression of the solution of octanoic acid and benzoic acid and compare with the accepted value (from the molecular formula)  Determine the molecular weight of an unknown

Set up for the freezing point determination.

Procedure: 1. Turn on the Vernier LabQuest2 and connect the temperature probe to Channel 1. 2. Set up two water baths in separate 400mL beakers. Add a 50/50 mixture of ice and tab water to fill one of the beakers ¾ full and fill the other ¾ full of hot tap water. Part I: Determine the Freezing Temperature of Pure Octanoic Acid 3. Place the ice water beaker on the base of the ring stand. Fasten the utility clamp to the ring stand. 4. Weigh approximately 4g of octanoic acid (record the precise mass) and transfer to an 18x150mm test tube. To get a good measurement, warm the test tube and contents in the hot tap water bath to around 30° before going to the next step. 5. Insert the temperature probe into the octanoic acid. Fasten the test tube to the utility clamp so it is above the ice water bath. Finally, click the green play button in the bottom left corner of the screen on the LabQuest2. 6. Lower the test tube into the ice water bath. Make sure the water level outside the test tube is higher than the octanoic acid level inside the test tube, as shown in Figure 1. 7. With a very slight up-and-down motion of the temperature probe, continuously stir the octanoic acid for the ten-minute duration of the experiment. 8. When the data collection is complete, use a hot water bath to melt the octanoic acid enough to safely remove the temperature probe. Carefully wipe any excess octanoic acid liquid from the probe with a paper towel or tissue. 9. The freezing temperature can be determined by finding the mean temperature in the portion of the graph with nearly constant temperature. a. Using the stylus on the LabQuest2, click and drag to highlight the plateau in the temperature curve. b. At the top of the screen, click the “Analyze” tab, then select statistics, and finally check the box that says “Temperature”. c. The mean temperature value for the selected data is listed in the statistics box on the graph. Record this value as the freezing temperature of pure lauric acid. d. Store the data from this run by clicking the filing cabinet icon on the right-hand side of the screen, just next to “Run 1”. You should now see “Run 2” listed as the title. Part II: Determine the Freezing Temperature of a Solution of an Unknown and Octanoic Acid 10. Add approximately 0.5g of benzoic acid (record precise mass) to the test tube containing the octanoic acid from part I. Seal the test tube with parafilm. 11. Return the test tube to the hot water bath for a short time until all solids are melted. Using a test tube holder, you may agitate the test tube periodically to hasten dissolution of benzoic acid. 12. Retrieve your test tube containing the octanoic acid-benzoic acid solution using a test tube holder. Remove the parafilm and repeat steps 5-9. 13. Weigh approximately 4g of octanoic acid and transfer to a clean labeled test tube. Repeat steps 11 and 12 but substitute 0.5g of the unknown in place of benzoic acid. Record precise masses of unknown and octanoic acid. 14. To print your plots, you will need to get them into excel. a. To do this, click the “Run 3” button on the right side, just left of the file cabinet icon. Change this to “All Runs.” Then click on “File” then “Export”. Plug in an USB drive and save this to the drive.

b. On your computer, open Excel first, then click the select “Open” on the left. From the options, choose “Browse” and navigate to your USB drive. You will need to change the file type in the bottom right hand side to “All Files (*. *)” then select your file. Once you open this, a new window will open. Click “Finish” in the bottom right of this window. c. Your data is now in excel. You will have to scroll down to find the second and third runs. Use this to make a plot for each run. Print and attach these graphs to your lab report. 15. At the conclusion of all data collection, heat the test tubes to melt the samples. Discard the liquified mixtures into a beaker provided in the hood. Wash the test tubes thoroughly with soap and warm water. Rinse the temperature probe with distilled water and wipe dry. When finished with the LabQuest2, please shut it down by clicking the home button (there is a physical button for this,) then select the “System” folder, then “Shut Down.”

Data Table Mass of octanoic acid (g) Mass of benzoic acid (g) Freezing temperature of pure octanoic acid (°C) Freezing point of the benzoic acid-octanoic acid mixture (°C) Mass of octanoic acid (g) (solvent for unknown) Mass of unknown (g) Freezing point of the unknown-octanoic acid mixture (°C)

Data Analysis 1. Calculate molality (m), in mol/kg, using the formula octanoic acid is 4.70°C·kg/mol.

∆ T f =k f m . The

kf

value for

2. Calculate the moles of benzoic acid solute, using the molality (obtained in step 1) and the mass (in kg) of octanoic acid solvent.

3. Calculate the experimental molecular weight of benzoic acid, in g/mol.

4. Determine the accepted molecular weight of benzoic acid from its formula, C6H5COOH.

5. Calculate the percent error between the experimental and accepted values for molecular weight of benzoic acid.

6. Calculate the molecular weight of the unknown solute.

Post-Lab Question: 1. What are the major sources of error in this experiment?

2. If the freezing point of the solution had been mistakenly read 0.5°C lower than the true freezing point, would the calculated molecular weight of the solute be too high or too low? Explain your answer.

3. A solution containing 0.200g of an unknown solute in 5.00g of octanoic acid freezes at 1.05°C below the normal freezing point of octanoic acid. What is the molecular weight of the unknown solute?...