Colligative Properties and Freezing-Point Depression

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Feb 20, 2024

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Oliveira 1 Colligative Properties and Freezing-Point Depression by Taylor Oliveira CHEM 142 – Principles of Chemistry II Department of Chemistry and Biochemistry University of South Carolina 01-09-2024

Oliveira 2 Abstract To determine the molar mass of an unknown substance mixed with dodecanoic acid, the freezing point of pure dodecanoic acid must be found first by measuring out a specific weight (in grams) of the substance and heating it to 60 degrees Celsius. A temperature probe attached to a lab quest will produce a cooling curve graph in which one can determine the freezing point. Similarly, by mixing the unknown solute into the dodecanoic acid solution, the freezing point of the solution can be determined by following a very similar process. Using the formula ΔT = i*K(f)*m, the molality of the solution can be calculated which then will lead to the molar mass of the solution being found. In this experiment, the average freezing point of pure dodecanoic acid came to be 43.7 and the unknown and dodecanoic acid solution came to be 40.0, leaving ΔT to be 3.7 (taken into account the absolute value). After calculations, the molar mass of unknown #1 came to be 105 g/mol; however this was not one of the solutes molar masses. The assumption could be made that benzoic acid with a molar mass of 122 g/mol would be unknown one as it is the closest, however a variety of factors could have affected the calculations to get a lower number. Introduction The objectives for the experiment conducted include calculating a freezing point depression to find the molar mass of one of the unknown substances provided: salicylic acid, camphor, or benzoic acid. The experiment also provides a deeper understanding of the van’t hoff factor and graphing a cooling curve based on the data gathered in the experiment. A solution's freezing point is lower than the solvent's pure freezing point. This indicates that freezing cannot happen unless a solution is lowered to a temperature lower than that of the pure solvent. Although it is independent of the sort, size, or charge of the solute(s) particles in the solution, the

Oliveira 3 freezing point of the solvent in a solution varies as the concentration of the solute in the solution does. When a pure solvent is added to a solution, its boiling and freezing points can alter. This can result in a drop in the pure solvent's freezing point and an increase in its boiling point. Compounds without ions that are merely molecules are called nonelectrolytes. In contrast, most powerful electrolytes are composed of ionic compounds, and almost all soluble ionic compounds are electrolytes. As a result, if it can be shown that the substance is uniform and not ionic, it is safe to conclude that it is a nonelectrolyte and attempt to solve this problem using a formula. The dodecanoic acid, which is made up of carbons, oxygens, and hydrogens, is not ionic, therefore giving a Van’t Hoff Factor of 1 which will be used into the formula to calcutta delta T. In this experiment, the unknown substance added to a solution of dodecanoic acid is aimed to be uncovered through calculating and graphing freezing point depressions for pure dodecanoic acid and the followed up by that same dodecanoic acid mixed with 1 gram of the unknown solute. The freezing point of the dodecanoic acid will be found first by heating it up to 60 degrees celsius and tracking its temperature decrease using a temperature probe attached to a lab quest for a given amount of time. This same process will be followed for tracking the temperature for the unknown solute and dodecanoic acid solution after they are mixed together and heated up to 55 degrees celsius. Each experiment will have two trials to get the average freezing points for more accurate results. Knowing how to determine freezing points from cooling curves, constructing graphs, and calculating delta T, molality, and molar masses are all vital factors for the success of the experiment. Trying to avoid errors and outside factors that skew results is also very important as accurate numbers are crucial in determining the molar mass of the unknown substance. The unknown substance we are trying to calculate is unknown #1.

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Oliveira 4 Procedure To conduct the experiment of the freezing point of pure dodecanoic acid, one 16x150 mm test tube, one 150-mL beaker, 1 hot water (70-80°C) bath (600-mL beaker and hot plate), 1 digital thermometer and lab quest, and 1 glass stirring rod, was all the equipment needed. To begin the experiment, a test tube was set in the 150-ml beaker which was tared to 75.1489g. Around 10-11 grams of dodecanoic acid must fill the test tube. 10.6191 grams was weighed out. The next step was to melt the dodecanoic acid by placing the test tube with the substance into the hot water bath and heating it. To help with the melting process, a glass stirring rod was used to push down the substance, however some substance stuck to the rod and solidified, therefore a chemistry wipe was used to get it off. Next, the test tube was removed from the hot water, then attached to a clamp over the hotplate and gently placed in the empty beaker, but not touching the beaker. To set up the lab quest to graph the temperature, the duration was set to 600s, rate to 2 sample/s, and interval to 0.5 s/sample. Then to begin measuring the temperature, the temperature probe was connected to the lab quest. Then, another clamp was attached to the hot plate support rod and the temperature probe, and the probe was placed into the solution in the test tube, but not touching the sides of the actual glass tube. When the temperature reached 62°C the green play button was pressed to record the graph. To collect the data after 600s, a flash drive was placed into the side of the lab quest and we selected file, export, name your file, and clicked OK. To get a more accurate average of the dodecanoic freezing point average, the test tube was placed back into the warm water bath to heat it up to 62°C again to graph the temperatures one more time, repeating the same steps from placing the tube into the hot water and so on. Part 2 of the lab was determining the freezing point of an unknown and dodecanoic acid solution and was conducted using the same equipment used in part 1. The first step was to weigh out 1 gram of the unknown

Oliveira 5 solution and its exact mass was recorded at 1.0034 g. Using the same dodecanoic acid sample from part 1, that testing tube was placed in the hot water bath and after it had all melted, the unknown substance was added into the testing tube. After adding the unknown, it was mixed into the sample using a glass stirring rod until fully dissolved. The solution would then be heated up to 55 degrees celsius in trials 3 and 4. The same steps were repeated the following steps from part 1 beginning with taking the test tube out of the bath and clamping it along with the temperature probe, except this time the lab quest had a duration set to 840s, rate to 2 sample/s, and interval to 0.5 s/sample. Two trials were again performed following the same steps from part 1 but using the 840s duration on the lab quest. Results and Discussion Figure 1: Pure Dodecanoic Acid Freezing Point Trial 1 Line Graph

Oliveira 6 Hypothesis test results: Variable n Sample Mean Std. Err. Z-Stat P-value Temperature (C) 1201 49.820316 0.16120234 309.05454 <0.0001 Figure 2: Pure Dodecanoic Acid Freezing Point Trial 1 Z-Statistic Figure 3: Pure Dodecanoic Acid Freezing Point Trial 2 Line Graph Hypothesis test results: Variable n Sample Mean Std. Err. Z-Stat P-value Temperature (C) 1201 49.527893 0.15867188 312.14033 <0.0001 Figure 4: Pure Dodecanoic Acid Freezing Point Trial 2 Z-Statistic

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Oliveira 7 Figure 5: Unknown and Dodecanoic Acid Solution Freezing Point Trial 3 Line Graph Hypothesis test results: Variable n Sample Mean Std. Err. Z-Stat P-value Temperature (C) 1681 43.917311 0.10977659 400.0608 <0.0001 Figure 6: Unknown and Dodecanoic Acid Solution Freezing Point Trial 3 Z Statistic

Oliveira 8 Figure 7: Unknown and Dodecanoic Acid Solution Freezing Point Trial 4 Line Graph Hypothesis test results: Variable n Sample Mean Std. Err. Z-Stat P-value Temperature (C) 1681 43.537597 0.11577227 376.06238 <0.0001 Figure 8: Unknown and Dodecanoic Acid Solution Freezing Point Trial 4 Z Statistic The graphs above show a cooling curve for the four trials performed. In figures 1 and 3, the graphs are extremely similar as the same procedure was performed for both graphs, this

Oliveira 9 would hypothesis test results for figures 2 and 4 to also be extremely similar. Two trials were performed for each experiment to get more accurate results. In figure 1, the freezing point for the dodecanoic acid was determined to be 43.7 as that is when the graph plateaued. In figure 3, the freezing point for the dodecanoic acid was determined to be 43.6 as that is when the graph plateaued. Taking the two averages would show that the freezing point for dodecanoic acid is 43.7, when rounding up. Other important results include the sample size both being 1201 and each graph was measured for 600 seconds. The seconds to graph on the lab quest must be this long to get an accurate freezing point so it is when the graph actually plateaus. The two graphs had slightly different means being 49.8 and 49.5 and slightly different standard errors being 0.161and 0.158. There could be a variety of reasons why the second graph had slightly lower numbers, but it is accurate as both temperatures had the sample size, started at 62 degrees celsius, and was taken over a length of 600s. In the second experiment to find the average freezing point for the data started to be collected when the dodecanoic acid and unknown solute solution was at 55 degrees celsius and was measured over a length of time at 840s. For a very similar reason, the length of time was 800s to get an accurate freezing point ( when the temperature plateaued. Figures 5 and 7 have extremely different graphs and figure 5 had a freezing point at 40.1 and figure 7 had a freezing point at 39.9, giving an average of 40.0 to be calculated for delta T. Figures 6 and 8 show the same sample size of 1681 but figure 6 had slightly higher statistics with a mean at 43.9 while figure 8 was 43.5. The standard error for trial 3 was 0.11 while trial 4’s standard error was 0.12. The begin calculating the molar mass of the unknown solute delta T was found by subtracting 43.7 - 40.0 which would be 3.7. 3.7 was then divided by K(f) given as 3.9 to give a molality of 0.95. To find moles solute, 0.95 m was multiplied by .011 kg solvent to give 0.1045 mol solute. Finally 0.001 g solute was divided by

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Oliveira 10 0.01045 mol solute to give a final molar mass of 105 g/mol. This however was not given as a molar mass of any of the unknown solutes given so the assumption could be made that the unknown was benzoic acid, but a clear decision cannot be made and therefore the experiment can be deemed inconclusive and soul be conducted again for a more accurate result. Conclusion As stated in the results, the molar mass was calculated to be 105 g.mol. Unknown #1 was the solute assigned to our group, but the unknown possible solutes were salicylic acid (Mm = 139 g/mol), camphor (Mm = 152 g/mol), and benzoic acid (Mm = 122 g/mol). As the calculations were correct, the result does not match any of the solutes provided, making the experiment null and having to perform it again to get accurate results. There are a variety of errors that occurred or could have occurred. First and foremost, the grams of dodecanoic acid was to be between 10-11 grams and we had 10.6191 grams, begging the question would the amount of grams affect the final calculation. Similarly, the unknown solute mass was to be 1.00 grams while the mass of our unknown solution was weighed to 1.0034. More importantly, during our second trial of experiment 1, our glass tube broke while boiling it back up 60 degrees celsius. This led us to redo the entire experiment from the start. The glass breaking was most likely caused by the clamp being too tight. Although the experiment was redone in its entirety leading to most likely accurate results, it is important to be noted. Secondly, while trying to push down the dodecanoic acid with a glass stirring rod to help it melt during experiment one, a little bit of the solution stuck to the rod, therefore taking it out of the solvent and the solution during experiment 2, as the same solvent was used in both experiments. This most likely altered the amount of grams in both solutions which could have then in turn affected the freezing points and the final molar mass calculated as it was inaccurate. Again, this experiment would have to be

Oliveira 11 performed again and I would try different methods to try and keep the solution in the tube which would hopefully lead to more accurate results and less errors during the experiment. The procedure accuracy could also be assumed not the greatest, but the freezing points received and graphs plotted were seemingly accurate along with accurate calculations with the numbers computed. References Freezing Point Depression. (n.d.). https://www.chem.purdue.edu/gchelp/solutions/freeze.html#:~:text=Freezing%20Point%2 0Depression&text=The%20freezing%20point%20of%20a,order%20for%20freezing%20to %20occur. Libretexts. (2023, February 27). Freezing Point Depression . Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_ Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties _of_Matter/Solutions_and_Mixtures/Colligative_Properties/Freezing_Point_Depression

Colligative Properties and Freezing-Point Depression

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