Module 4 Week 1 - Group Worksheet

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Michigan State University *

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Chemistry

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Dec 6, 2023

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M ODULE 4 - Q UANTITATIVE T ESTING - H OW MUCH STUFF IS T HERE W EEK 1 G ROUP A SSIGNMENT The background for this module available on D2L. Please read it! In this group assignment, you will do the following: ● determine the concentration of MnSO 4 from gravimetric analysis data (Part A) ● consider the relative advantages and disadvantages of using gravimetric analysis to determine the concentration of an unknown solution (Part A) ● the mathematical relationship between absorbance and concentration (Part B) ● apply critical thinking to determine (experimentally and mathematically) the concentration of an unknown solution using spectroscopy and a calibration curve when your solution is too concentrated or too dilute. (Part B) S AFETY 1. Scroll forward in this activity and look for chemical names. For each, look up the relevant safety information ( https://chemicalsafety.com/sds-search/ ), and complete the table below. You should add additional rows to the table if needed. Chemical name (not formula) Flammable? Reactive? Health Hazard? First Aid Measures Needed protective equipment Manganese(II) Sulfate not flammable, is reactive, is a health hazard If inhaled: go to fresh air and keep at rest in a position comfortable for breathing. If in eyes: rinse cautiously with water for several minutes. remove contact lenses if present. If on skin: wash with plenty of water. If swallowed: rinse mouth. Call a POISON CENTER or physician if you feel unwell. Wear protective gloves, protective clothing, and eye protection. Wash hands thoroughly after handling Cobalt nitrate Intensifies fire, cause skin burns, cancer, may cause birth defects, toxic to aquatic life Wash thoroughly after handling, if in eyes rinse cautiously with water, if inhaled go to fresh are, if swallowed rinse mouth and call poison center wear protective gloves, and goggles [Sodium Chloride] [not flammable, not reactive, it can be a [Insert Your Answer Here] [Insert Your Answer Here] 1

health hazard] 2. Scroll forward through this activity and consider what kinds of procedural hazards you would encounter if you were going to do this activity live (hot glass, containers which could spill, glass which could be dropped and shatter, etc.). For each possible hazard, identify ways that the risk could be minimized. You should add additional rows to the table if needed. Procedural Hazard What could go wrong? If it does go wrong? What should you do? What can you do to proactively prevent this from happening? touching hot equipment/ misplacing things on hot equipment You could burn yourself or set something on fire keep things far away from hot equipment/ turn off when not in use. Use gloves. Covering all container and bunsen burners you could bump into it and cause a spill. If this occurs, using gloves clean up the counter and make sure nothing is contaminated. also to ensure there are no splashes from the bunsen burners. always cover containers when you are not using them and keep them away from the ledge. P ART A: A NALYSIS OF G RAVIMETRIC A NALYSIS E XPERIMENT D ATA In this experiment, you “were given” access to a dark purple solution of MnSO 4 with an unknown concentration. You heat the solution using the set-up below (Figure 1) with a known volume of solution in a bowl (called an evaporating dish). Figure 1 . This is the set-up for gravimetric analysis. From left to right: a striker, a ring stand, a bunsen burner, a clay triangle, metal tongs, and another clay triangle. The bowl to hold the solution is not shown. You weighed the bowl before and after heating the solution with a lot of accuracy. You heated the solution until all of the water was boiled off and only a white, powdery solid was left in the bowl. Even after it looked like you had evaporated off all the water, you heated and weighed the bowl several times more. 2

The instructions for doing the gravimetric analysis are provided in the appendix found at the end of this worksheet. Here’s a short video demonstrating how to run the gravimetric analysis experiment: https://mediaspace.msu.edu/media/Gravimetric+Analysis/1_j7kmo044 1. Why is it important to heat, cool, and weigh the bowl several times even after it looks like all the water is gone? It is important to heat, cool, and weigh the bowl several times so that you can check and make sure that the mass of the bowl is consistent and ensures that the water is all gone. If there is extra water then you will get the weight of the manganese wrong and the data and concentration will be incorrect. After a lot of slow work, here is the data that your group recorded: Dish 1 Dish 2 Dish 3 Dish 4 Dish 5 Dish 6 Dish 7 Dish 8 Dish 9 Dish 10 Mass of empty dried evaporation dish (g) Trial 1 27.2185 30.5860 25.6126 31.5034 30.1670 26.4605 113.592 27.2779 31.7560 112.3335 Trial 2 27.2137 30.5806 25.6073 31.5010 30.1664 26.4579 113.589 27.2778 35.7608 112.3405 Trial 3 27.2164 30.5747 25.6091 31.5057 30.1677 26.4614 113.609 27.2783 35.7609 112.3450 Volume of MnSO4 solution added (mL) 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Mass of dried evaporation dish with MnSO 4 (g) Trial 1 28.8980 32.3376 27.3465 33.1345 31.9201 28.2663 115.4459 29.0475 33.3420 114.117 Trial 2 28.8825 32.3036 27.3265 33.1388 31.9030 28.1414 115.3930 29.0113 33.3490 114.020 Trial 3 28.8840 32.2991 27.2967 33.1275 31.8941 28.1437 115.2790 29.0239 33.3469 114.008 Trial 4 33.1296 28.1387 115.2710 113.976 Trial 5 28.1281 115.2270 113.973 Trial 6 28.1089 115.2260 Trial 7 28.1011 **Note: If a dish is weighed while still hot, the mass will appear to be lower than it actually is. 2. Looking at the values for the masses of the evaporation dish with the dried manganese sulfate: Why was the dish and solid in dish 1 heated and weighed three times, when the samples in dishes 6, 7, and 10 were heated and weighed many more times? Do you think that was wise? This works because the goal is to find constant mass. This is to show that the mass is staying relatively close. The difference to ensure constant mass is ±0.005 g. This confirms all the moisture is gone. The amount of trials may also vary if the person doing the experiment wishes to do more trials to confirm the weight. For example, dish 6. The weight was fitting at trial 3 but then they continued. 3. Comment on the quality of the data. Was the criteria for constant mass met in all cases? Were the values recorded reasonable? 3

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No, there are some trials in which the trials are continued even though the law of constant mass is met. The law of constant mass was not met for dishes 3, 5, 6 and 8. This is because the final difference between the last two trials for these dishes is not + or - 0.005g. There is another error in dish 9 because the mass from the empty dish is more than that of the dish with MnSO4. This is incorrect because it is not possible to lose weight when you are adding a mass. 4. What is the concentration (in molarity) of the MnSO 4 in the solution? Report an average molarity and a standard deviation. If you forgot how to calculate a standard deviation, refer back to the Accuracy and Precision Module. Include a sample calculation for the calculation of the molarity for one of the trials. Sample calculation for molarity determination: [ Dish 2 Trial 1 molarity: .012/.005=2.327] Average molarity of MnSO 4 : [1.86 plus or minus .12] Standard deviation: [.006] 5. What do you see as some of the advantages and disadvantages of the gravimetric analysis method for determining the concentration of an unknown solution? List at least two of each. Advantages: ● The concentration is accurate and has a very precise result ● This is a visual process and you are able to see the different steps. Disadvantages: ● very long process and time consuming ● there could be a lot of error P ART B: D EEPER E XPLORATION OF THE E XPERIMENTAL I SSUES WITH Q UANTIFYING S OLUTION C ONCENTRATION U SING S PECTROSCOPIC M ETHODS 6. As a group, write out the relationship between concentration, path length, and absorbance using information provided on the week 1 overview page. . Mathematical relationship : A = ε l c. Concentration and absorbance have a (select one) direct relationship. What kind of graph can you make of these relationships? calibration plot,/scatter plot What is plotted on the x-axis? [concentration (mol/L)] What is plotted on the y-axis? [absorbance] Now, let’s explore how we can use spectroscopy and the relationship between absorbance and concentration to determine the amount of material in an unknown solution. Central to this work is the graph of the relationship between the concentration, path length, absorbance, and molar absorptivity variables. 7. There are two names for this kind of graph, one more general and the other more specific. What 4

are they? calibration plot and beer lambert plot 8. Let’s say that in an experiment, your group found that a solution of Co(NO 3 ) 2 ( a beautiful pink- orange color ) had an absorbance of 0.76 using a cuvette with a path length of 1cm. When a Beer Lambert Plot was prepared, absorbance plotted on the y-axis and the concentration of the cobalt nitrate (in mM) on the x-axis, the equation of the best fit curve (linear) was found to be ( y =0.0047 x + 0). What is the unknown concentration of this Co(NO 3 ) 2 solution? Given best fit equation: a = 0.0047c + 0 unknown variable: concentration of Co(NO3)2 Work to solve for unknown variable: c = a/0.0047 Concentration of Co(NO 3 ) 2 : 161.7 mM I SSUE #1: S OLUTION IS T OO C ONCENTRATED In theory, we should only ever make concentration determinations for solutions that fall between points on your calibration plot, as we can’t have confidence that your linear fit accurately expresses the relationship between concentration and absorbance infinitely (in fact they never do). Another way of saying this is that we can interpolate using our calibration curves but we cannot extrapolate. Video (2:42”) about interpolation and extrapolation: https://www.youtube.com/watch?v=c4_MJg_c49k Figure 2 . Scatter plot of data with a linear best fit. Dots represent measured data points. Interpolation is using the linear fit to estimate a value that is between measured data points. Extrapolation is to estimate a value that is beyond a measured data point. Figure copied from: http://pillars.che.pitt.edu/student/slide.cgi?course_id=12&slide_id=17.0 . We can, however, use Beer-Lambert plots to find concentrations outside of the measured range by diluting your unknown sample to decrease it’s absorbance by a known amount and then back calculating the true concentration from the measured absorbance. 5

For example : Imagine that you had a beautiful, pink solution of Co(NO 3 ) 2 of unknown concentration. We will call this Solution A . You put a sample of Solution A into a 1 cm cuvette. Upon measuring the absorbance of your sample, you see that the instrument gives an absorbance reading of 2.6, above your maximum calibration plot point. [Note: A reading greater than 2 means that it is absorbing over 99% of the photons that are entering the solutions, which you can imagine is then hard to measure precisely.] This reading of 2.6 is not helpful. You decide to dilute Solution A. You take 1.0 mL of Solution A, transfer it into a 100 mL volumetric flask, and add water until you reach a total volume of 100.0 mL. You call this Solution B . 9. Sketch out how you would do this dilution experimentally. How much of what is going where? Be intentional (be specific!) about what glassware you use. [ ] This new solution (Solution B) has a lower concentration of your unknown sample. You do not know the concentration of Solution B, but you do know that it’s 1/100th of the more concentrated unknown solution. 10. You take some of this diluted solution (Solution B) and measure its absorbance. You get an absorption reading of 0.54. Using your best fit line from question 8, what is the concentration of the diluted solution, Solution B? Show your work. a = 0.0047c + 0 0.54 = 0.0047 c + 0 c = 114.8 mM 6

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11. What was the concentration of the original solution, Solution A? Show your work. m1v1 = m2v2 M1 = m2v2/v1 M1 = 0.1148 (100) /(1) M1 = 11.5 M I SSUE #2: U NKNOWN S OLUTION IS T OO D ILUTE Another situation might be where you want to measure the original concentration of Solution C. Solution C is also made of Co(NO 3 ) 2 but has a different concentration than Solution A. You place a sample of Solution C in a 1 cm cuvette and find that the measured value is bouncing around quite a bit between 0.04 and 0.08. This is quite common when using an instrument due to the natural level of “noise” in the detector. This “bouncing around” means that the value is not directly usable. One option is to use a cuvette (container) with a longer path length. This would provide more opportunities for the solute molecules to absorb photons. 12. If you take that same original unknown Solution C and put it in a cuvette that has a 4.0 cm path length and find that the absorbance of the solution becomes 0.27, what would be the concentration of the solution? Again, use the best fit line from question 8. Show your work. y = 0.0047x + 0 0.27 = 0.0047x + 0 x = 5.7 x 10 ^1 mM I SSUE #3: S OLUTE C HOICE 13. UV/Vis Spectroscopy is not a suitable technique for measuring the concentration of NaCl in solution. Why do you think this is the case? This is because UV/V doesn’t work because it is colorless and it doesn’t absorb any light . Because NaCl is colorless you would not be able to collect any data. Also because it doesn’t have the correct amount of measuring it only goes 200 nm to 800 nm 14. For a solution of NaCl, what would you expect the % transmittance and absorbance to be? 100% transmittance 0% absorbance Submit a PDF of this completed worksheet to D2L. The easiest way to make a PDF of a Google doc is outlined here: https://9to5google.com/2019/11/09/create-google-docs-pdf-document/ . Also submit the excel sheet you used to do the calculations of the concentration of manganese sulfate using the gravimetric method. L OOKING A HEAD : W EEK 2 Thus far your determination of how much stuff is in a solution has been theoretical (in the simulations) or by analyzing data (gravimetric analysis) . Next week, your group will conduct an experiment in the lab 7

using spectrometry to quantify the amount of manganese in a provided solution. The protocol for next week's experiment will be provided for you as a separate document under the Module 4 Week 2 section of our course D2L page . Further information and expectations relevant to week 2 will also be available under the Module 4 week 2 section on D2L. A PPENDIX G RAVIMETRIC DETERMINATION OF M N SO 4 CONCENTRATION IN SOLUTION . A solution of Manganese sulfate is dried to remove all the solvent. At this time MnSO 4 .H 2 O is formed. The solid is further heated to drive off the remaining water. The equations for these changes are as follows: 2 − ¿ ( aq ) + n H 2 O ( aq ) ∆ ( 100 ℃ ) → MnSO 4 .n H 2 O ( s ) 2 + ¿ ( aq ) + SO 4 ¿ Mn ¿ MnSO 4 .n H 2 O∆ ( 400 − 500 ℃ ) → MnSO 4 ( s )+ n H 2 O ( g ) P REPARATION OF E VAPORATION DISH Prepare the evaporating dishes to be used to evaporate the sample over the steam bath. The dishes must be dried to constant mass before using (the masses obtained must be within 0.005g of each other. 1. Clean and dry an evaporating dish and mark the dish with a pencil. 2. Place the dish on a wire gauze resting on an iron ring. Heat the dish with a Bunsen burner until all the condensed moisture has been driven off. This should take about 5 minutes. 3. Allow the dish to cool to room temperature and weigh and record the mass. a. Do this by using crucible tongs to handle the dish and a wire gauze to support it. This will prevent you from getting oil on it from your fingers. 4. Repeat the heating process until your weight is constant to ±0.005 g indicating that all the moisture has been removed. E VAPORATION AND DRYING OF SAMPLE 1. Pipette 5.00 ml of the manganese sulfate solution into each of the dried evaporating dishes. Note that for the best accuracy, you should do this using a volumetric pipet . 2. Place them on the steam bath (on the hot plate) and heat uncovered until dry. 3. Remove the evaporating dishes from the steam bath and place each of them on a clay triangle. 4. Heat gently, uncovered, for a few minutes with the Bunsen burner to remove excess moisture. 5. When the solid appears dry, heat the dish strongly for about 5 minutes. 6. Remove the flame and allow the dishes to cool, then weigh and record the mass. 7. Repeat the process of heating over the burner, cooling and weighing until constant mass is achieved (indicating that all the moisture has been removed.) 8