IMF Freezing Point_F23-1

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Dec 6, 2023

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Intermolecular Forces: Freezing Point Depression Fall 2023 Directions: This is a FULL lab report. You are required to write an Abstract and Discussion section along with all other required material 10/6/23 The objective of this lab was to measure the freezing point of a pure substance and then its freezing point depression when an unknown substance is added to it. From the experiment, The freezing point and freezing point depression. The values were further used to find molality, moles of solute and molar mass to find the identity of the unknown. The identity of the unknown was found to be… LAB DATA SECTION: ( 20 points) Page 1 of 9

Intermolecular Forces: Freezing Point Depression Fall 2023 Lab Data This information (blank) needs to be in your lab notebook before you come to lab. You will fill in the data directly into your notebook as you complete the lab. Extend the table for as many rows as necessary. Mass of test tube + stir bar : __ _ 16.38g___ Mass of test tube +stir bar+ TBA: __27.12g___ Mass of TBA: ___10.74____ Unknown Number/Identifier: ___2____ Solution I Mass of test tube + stir bar+ TBA + 1st addition of unknown: __27.62g____ Mass of unknown solid in Solution I: __0.50g____ Solution II Mass of test tube + stir bar + TBA + 2 nd addition of unknown: ___28.12g___ Mass of unknown solid in Solution II: ___0.50g___ Replicate the table below in your lab notebook in order to record the temperature of each solution. You may need to extend table so leave room. Alternatively, record the temperature using Excel (for this option you will need to bring your laptop). Time (minutes) Pure TBA Temp. ( o C) Solution I Temp. ( o C) Solution II Temp. ( o C) 0.5 30.9 38.9 40.7 1 34.6 40.3 42.5 1.5 37.9 38.5 42.8 2 40.1 38.7 43.4 2.5 37.1 35.6 39 3 36.5 34.6 37.3 3.5 35.7 33.2 35.4 4 35 32 33.9 4.5 34.4 31.1 32.6 5 33.5 30.1 31.5 5.5 33.1 29 30.5 6 28.1 25.8 29.6 Page 2 of 9

Intermolecular Forces: Freezing Point Depression Fall 2023 6.5 26 23.8 25.2 7 25.5 22.3 21.3 7.5 25.8 21.3 18.9 8 25.1 20.7 17 8.5 24.5 20.2 15.9 9 23.6 20 15 9.5 22.9 19.8 14.4 10 22.2 19.6 13.9 10.5 21.6 19.4 13.6 11 20.8 19 13.2 11.5 20.1 18.7 12.9 12 19.2 18.4 12.5 12.5 18.4 18 12.2 13 17.8 17.5 12.1 13.5 17.2 16.9 11.8 14 16.7 16.3 11.2 14.5 16.2 16.8 10.7 15 15.8 16.5 12 15.5 15.2 15.8 11 16 14.8 15.3 10.4 16.5 14.2 14.6 10.4 17 13.8 9.5 17.5 13.5 10.9 18 13.2 10.2 18.5 13 9.5 19 12.8 9.1 19.5 13 9.2 Results The below section does not need to be in your lab notebook before lab . You will complete this Page 3 of 9

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Intermolecular Forces: Freezing Point Depression Fall 2023 section after finishing all calculations . Skip to calculations section and then come back here to fill in the results once finished. 1. Freezing points (from graphs) Pure TBA: ___19___ Solution I: __20.7____ Solution II: ____15.9____ 2. Observed Δ T f for: Solution I: _________ Solution II: ____________ 3. Molality of: Solution I: _________ Solution II: ____________ 4. Moles of unknown solute in: Solution I: _________ Solution II: ____________ 5. Molar Mass of Unknown: Solution I: _________ Solution II: ____________ Average Molar Mass of Unknown ______________ 6. Identification of Unknown (Use the unknown list from IMF Part A to choose from): _____________ 7. Percent Error for Molar Mass: ________________ CALCULATIONS (20pts): Directions: below is a helpful guide for the calculations you must complete. Complete these and upload images of your calculations either into this section directly or separately on canvas to get your results. You do not have to do any extra credit, it is optional. 1. Determine Freezing Point (Tf) for TBA, Solution I, Solution II: a. Estimate (qualitatively) the freezing point (Tf) for all solutions visually (ie from the point Page 4 of 9

Intermolecular Forces: Freezing Point Depression Fall 2023 where liquid phase transitions to solid. This should be clear from your graph as a change in the slope. b. Extra Credit: Determine (quantitatively) the Freezing Point (Tf) for TBA, Solution I, Solution II: (+10pts) Please watch the following excel tutorial for instructions on how to fit your freezing point curves correctly in Excel. If you just "eye/estimate" the temperature, you will get the wrong Tf and points deducted. Excel Demo by Steven Poteet, UVA: https://www.youtube.com/watch?v=RUHFzEpZBec Use Excel to graph temperature (y-axis) vs time (x-axis) for each solution above. Label the graph titles and axes correctly. From your graphs determine the freezing point of each substance: ● In Excel, connect each cooling curve with two straight lines (one for the liquid portion, and one for the solid portion) ignoring points involving super cooling. ● Be sure to add the treadline (y=mx+b) for each line to the cooling graph ● The freezing point (Tf) is the point where the two lines EXACTLY intersect. ● Therefore, to determine Tf, you must equate the linear treadlines (the y=mx+b) equations of both your lines determined using Excel. This is the exact intersection point, where both lines have the same y and x value. 2. Calculate the freezing point depression ( Δ T f ) for Solution I and Solution II. ● Calculate the Δ T f values relative to the pure TBA ● Recall, the freezing point depression is how much the freezing point has lowered due to addition of the solid unknown relative to freezing point of pure TBA. 3. Determine Molar Mass of the unknown in Solution I and Solution II. ● Using the k f for TBA calculate the molality of each solution (assuming i = 1). The mass of solvent is the mass of the original TBA (expressed in kg). ● Using the molality of each solution and the Δ Tf, calculate and the moles of unknown solute for each solution. ● Calculate the molar mass of the unknown using the above information. Recall that molar mass is moles substance/gram substance. You can use the equation that relates ΔTf to molality and then use this to find molar mass ● Calculate the average molar mass using your molar masses determined from Solution I and Solution II (these should be similar since it is the same unknown!) ● Using the unknown list in the background, determine the identity of the unknown based on the Page 5 of 9

Intermolecular Forces: Freezing Point Depression Fall 2023 average molar mass you obtained. (You may have to look up the molar masses for these compounds first) ● Based on your determination of unknown, calculate the % error in molar mass relative to the true molar mass. Insert any Calculations and any Excel Graphs Here. Page 6 of 9

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Intermolecular Forces: Freezing Point Depression Fall 2023 DISCUSSION AND CONCLUSION (20 points): Discussion/conclusion statements should be a short essay (~1 page ) that anyone could read and quickly understand what you determined (the major results ), the scientific background that explains these results, if the results make sense given the experiment background , how precise and/or accurate your results were, sources of error, potential improvements to the experiment and applications of the experiment to a broader topic. For example, do not simply state that the objective of the experiment was met but rather state what the actual result was. Do not repeat all the procedures from the experiment but be clear when you discuss sources of error and improvements. For this experiment, address the following at a minimum withing the broader context of your discussion. (5 points). Discuss the molar mass obtained and identity of your unknown from based on the freezing point depression. Are you confident in the identity of the unknown referencing % error? (5 points) Did you see a difference in freezing point between solution I and solution II? Why/Why not ? Should there be a difference in freezing point? Explain. (5 points ). Do you see any possible sources of error in your data? Based on this, discuss improvements to the experiment. - (5 points). find a real-world example related to any topic covered in this experiment, discuss it briefly and include a citation. Possible sources of error include Delete the above directions and insert Discussion/Conclusion page here POST-LAB QUESTIONS (10 points): Directions: Answer the following questions either in your lab notebook (re-create graphs) or on paper. Page 7 of 9

Intermolecular Forces: Freezing Point Depression Fall 2023 Scan and insert your answers below or attached as a sperate file with your submission on canvas. Only handwritten answers will be accepted for this Post-Lab Question. In the following experiment, the molar mass of a nonvolatile and nonelectrolyte unknown solid was determined via freezing-point depression. In the experiment the pure solvent used froze at 30 °C and had a known molal freezing-point depression constant, Kƒ . The following materials were also available. • test tubes • stirrer • pipet • stopwatch • graph paper • thermometer • balance • beaker • ice • hot-water bath 1. (2 points). Using the two sets of axes provided below, sketch cooling curves for (i) the pure solvent and for (ii) the solution as each is cooled from 50 °C to 0.0 °C Pure Solvent Solution 2. (6 points total). Information from these graphs may be used to determine the molar mass of the unknown solid. (i) Describe the measurements that must be made to determine the molar mass of the unknown solid by this method Mass of the unknown, mass of solvent, freezing point of the pure solvent, and freezing point of the solution (ii) Show the setup(s) for the calculation(s) that must be performed to determine the molar mass of the unknown solid from the experimental data ΔT ❑ f =( Tf solvent − Tf solution ) molunknown = ΔTf ,kg of solvent Molarmass = gunknown / molunknown (iii) Explain how the difference(s) between the two graphs can be used to obtain information needed to calculate the molar mass of the unknown solid. Page 8 of 9

Intermolecular Forces: Freezing Point Depression Fall 2023 The horizontal part of the graphs are different meaning they have different freezing points. 3. (2 points). Suppose that during the experiment a significant but unknown amount of solvent evaporates from the test tube. What effect would this have on (he calculated value of the molar mass of the solid ( i.e. , too large, too small, or no effect)? Justify your answer. The value of the molar mass would be too small because if the unknown solvent is too big the mol of unknown would be big making the molar mass smaller. Page 9 of 9

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