Determining the Identity of an Unknown Metal PDF

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Determining the Identity of an Unknown Metal Bryanna Tanase CHM 2045L-019 TA: Jake Mayers October 19, 2016

Introduction Background: The ability to determine an unknown substance by its physical properties is an important skill for life, used both professionally and in everyday situations. For example, imagine if someone spilled a mysterious liquid on their shirt and wanted to identify what it was. They would examine the stain’s physical properties (color, size, smell etc.) and then come to a conclusion about what they spilled on their shirt. On a larger scale, if there was a new form of bacteria that was causing harm to both humans and animals, scientists would need to take a sample of the bacteria, look at the size, shape, color etc., and go on performing a series of chemical tests to be sure of its identity, just as done in this experiment. Theory: The key concepts needed in this experiment were a clear understanding of solubility, pH, titration, and gravimetric analysis. Hypothesis: Based on the fact that the unknown compound has a basic pH, is soluble in water, and has a low conductivity, it is reasonable to assume that its identity is sodium acetate Objectives: The objectives for this experiment is to perform a series of qualitative and quantitative tests (pH, solubility, etc.) on an unknown compound, and use that data to come up with an idea of the identity of the compound. Then, compare the properties of known metals to those results to determine its true identity. In order to perform this experiment correctly, one must have knowledge of solubility rules, pH, concentration, how to use instruments such as a pH meter and conductivity probe, how to make solutions, and how to perform gravimetric and volumetric analysis.

Methods Materials  25 ml graduated cylinder  50 ml/100 ml beakers  balance  Bunsen burner  Burette  Conductivity meter  pH meter Part 1: a. pH test- 0.5 grams of the unknown was dissolved in deionized water to create a 0.1 M solution. Then a pH meter was dipped in the solution and the resulting pH recorded. Four trials were conducted to ensure reproducibility of results.  To use the pH meter, rinse the probe with deionized water. Then place the meter in the solution, making sure the probe is completely submerged, and record the pH as shown on the meter. Make sure to rinse the probe in between trials to avoid contamination. b. Conductivity test-0.5 grams of the unknown was dissolved in deionized water to create a 0.1 M solution. A conductivity meter was then dipped into the solution and the resulting conductivity recorded. Four trials were performed to ensure reproducibility of results.

To use the conductivity meter, rinse the probe with deionized water. Then dip the probe into the solution, making sure it is completely submerged, and record the result as shown on the meter. Make sure to rinse the probe in between trials to avoid contamination c. Flame test- A wire loop was dipped into a 0.1 M solution of deionized water and the unknown and then placed in the blue part of the Bunsen burner flame. The color of the flame was observed and recorded. Three trials were performed to ensure reproducibility of results d. Solubility test- 0.5 grams of the unknown was dissolved in deionized water to create a 0.1 M solution. Then ethanol was added to the solution to see if the unknown would dissolve or form a precipitate. Four trials were conducted to ensure reproducibility of results. The solubility test was also performed using acetone and sodium nitrate to confirm speculations about identity.  To determine whether to use ethanol or acetone in this test, look up a table of solubility rules and use the ones for the possible identity of your unknown. Part 2: a. pH test- The pH of sodium carbonate, sodium nitrate, and sodium acetate were tested by making a solution of 0.5 grams each with deionized water. A pH meter was dipped into the solution and the pH recorded. Three trials were conducted to ensure reproducibility of results, then the average taken and compared with results from the previous week.  To use the pH meter, rinse the probe with deionized water/ Then place the meter in the solution, making sure the probe is completely submerged, and record the pH as shown on the meter. Make sure to rinse the probe in between trials to avoid contamination b. Conductivity test- 0.5 grams of sodium acetate, sodium nitrate, and sodium carbonate were made into a 0.1 M solution, then a conductivity meter was dipped into each and the conductivity recorded. Three trials were conducted to ensure reproducibility of results.  To use the conductivity meter, rinse the probe with deionized water. Then dip the probe into the solution, making sure it is completely submerged, and record the result as shown on the meter. Make sure to rinse the probe in between trials to avoid contamination c. Solubility test- 0.5 grams of sodium carbonate, sodium nitrate, and sodium acetate were each made into a 0.1 M solution. Silver nitrate was added to each solution to see if it would form a precipitate or not. Three trials were conducted to ensure reproducibility of results.  To determine whether to use ethanol or acetone in this test, look up a table of solubility rules and use the ones for the possible identity of your unknown. d. Titration- 10 ml of 0.5 M sodium acetate was titrated with 16.67 ml of 0.3 M hydrochloric acid using methyl orange indicator because hydrochloric acid is a strong acid and sodium acetate is a weak base. The pH was recorded once the solution reached the equivalence point. The titration was performed after all other tests to prove speculations about identity  To perform a titration, first make solutions out of the desired titrant and analyte. For example, let’s say the titrant is NaOH, the analyte is HCl, and 



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you want to make a 0.1 M solution of both. For the NaOH solution, weigh out 0.4 grams NaOH and place it in a volumetric flask. Then add 100 ml of water and stir with a stirring rod until the NaOH is dissolved. To prepare the HCl solution, add 20 ml 0.1 M HCl to a clean and dry Erlenmeyer flask. Then add 3 to 5 drops phenolphthalein to the flask (since the titration is between a strong acid and strong base). Once both *solutions are made, the titration procedure can begin. (Anderson, Figueroa, Lykourinou) To set up the titration, first rinse a funnel and burette with water. Then clamp the burette vertically to a ring stand and place the Erlenmeyer flask with the HCl (analyte) underneath it. Rinse the burette with less than 5 ml of NaOH (titrant) solution and let it run into an empty beaker for waste, closing the stopcock on the burette once all the liquid has drained into the waste beaker. Fill the burette to the 0 ml mark with NaOH using a funnel. Once the burette is filled, add the NaOH to the HCl one ml at a time, swirling the flask to ensure proper mixing, and check and record the pH of the solution after each addition. Add one ml until the solution turns pink, and then once it is pink, reduce the amount added to 0.5 ml until the color stays but then turns clear. When the pink color stays, take the volume of the NaOH in the burette to determine the volume of NaOH needed to reach the neutralization point. (Anderson, Figueroa, Lykourinou) Depending on the compounds you are titrating with and their acidic or basic properties, one may have to create solutions with a different concentration or use a different indicator, but the overall procedure remains the same

Safety: Goggles, gloves, closed toed shoes, and lab coat must be worn at all times during all times during this experiment. Only use small amounts of chemicals and dispose of waste in the proper container. Wash hands thoroughly before and upon completion of experiment. Clean glassware before and after use. Chemical Information 1. CaCl2- do not ingest, inhale, or put in contact with skin, eyes or nose. Keep sealed in a well ventilated area and always wear PPE 2. NaNO3- do not ingest, inhale, or put in contact with skin or eyes. Prolonged exposure may cause ulcers and burns. Keep away from heat, alkalis, and bases and store in a well ventilated place. Wear PPE at all times 3. (NH4)2SO4-Do not ingest, inhale or put in contact with skin or eyes. Keep away from oxidizing agents and heat and wear PPE at all times. 4. NH4Cl- do not ingest, inhale or put in contact with skin or eyes. Keep away from oxidizing agents, bases, and alkalis. Seal and store in a well ventilated place. 5. MgSO4-do not ingest, keep away from skin and eyes, always wear PPE 6. Ca(NO3)-do not ingest, inhale, or put in contact with skin or eyes. Prolonged exposure may cause burns and ulcers. Keep locked away from heat, sunlight, and oxidizing agents. Wear PPE

7. Na2C2O4- do not ingest, inhale, or put in contact with skin and eyes. Keep away from heat, evaporate residue under fume hood. Keep away from oxidizing agents and store in a well ventilated area. Wear PPE 8. Na2CO3-do not inhale, ingest, or put in contact with skin or eyes. Separate from acids and keep sealed in a well ventilated area. Wear PPE 9. CH3CO2Na- corrosive, do not ingest, inhale, or put in contact with skin or eyes. Never add water to it and keep dry, away from oxidizing agents. Wear PPE 10. NaCl- do not inhale, ingest, or put in contact with skin and eyes. Separate from oxidizing agents and acids. Keep sealed in a well ventilated area. Wear PPE. 11. HCl- corrosive, do not inhale, ingest, or put in contact with skin or eyes. Never add water to it and keep locked up and dry. Wear PPE 12. C2H6O- extremely flammable. Do not inhale, ingest, or put in contact with skin or eyes. Store outside in a well ventilated place. Wear PPE. 13. (CH3)CO- do not inhale, ingest, or put in contact with skin or eyes. Keep locked up away from heat, oxidizing agents, and alkalis. Store in a well ventilated area. Wear PPE,

Results Week 1 Conductivity Test Trial 1 Trial 2

2.65 microsiemens 1.85 microsiemens

Trial 3

2.58 microsiemens

Trial 4 Average pH test Trial 1 Trial 2 Trial 3 Trial 4 Average

2.93 microsiemens 10.01 microsiemens 8.5 8.0 7.8 7.9 8.05

Flame Test Trial 1 Trial 2

Yellow-Orange Yellow-Orange

Trial 3

Yellow-Orange

Trial 4

Yellow-Orange

Solubility Test Trial 1

Insoluble

Trial 2 Trial 3

Insoluble Insoluble

Trial 4

insoluble

Week 2 Results pH Trial 1 Trial 2 Trial 3 Average Conductivity Trial 1 Trial 2 Trial 3 Average Solubility Trial 1 Trial 2 Trial 3

Na2CO3 11.1 11.1 11.1 11.1 Na2CO3 19.8 microsiemens 19.47 microsiemens 21.2 microsiemens 46.3 Na2CO3 Insoluble insoluble insoluble

Titration Results Trials Acid volume 1 16.67 ml 2 16.67 ml 3 16.67 ml

Base volume 10 ml 10 ml 10 ml

CH3COONa 9.6 9.6 9.5 9.56 CH3COONa 6.65 microsiemens 6.59 microsiemens 6.44 microsiemens 6.56 CH3COONa Soluble Soluble Soluble

Indicator drops 3 3 3

NaNO3 7.5 7.1 7.1 7.23 NaNO3 8.6 microsiemens 26.4 microsiemens 11.85 microsiemens 38.95 NaNO3 Insoluble Insoluble insoluble

Acid concentration 0.3 M 0.3 M 0.3 M

Base Concentration 0.5 M 0.5 M 0.5 M

Calculations  To find the average of all trials, add the results of each trial together and divide by the number of trials o 19.8 + 19.47 + 21.2/3 = 46.3  To find concentration of the unknown, which has a mass of 0.5 grams, divide mass by volume, do the same for known compounds o 1g/.20 L x 100 = 0.5%  To find the volume of acid and base needed for titration, use M1V1=M2V2, where M1 is the molarity of the acid or base, M2 is the molarity of the unknown, and V2 is the volume of the solution containing the unknown o (3 M HCl)(x) = (.5 M)(100 ml) o x= 50/ 3 o x= 16.67 ml HCl

Discussion Part 1: For the conductivity test, once all trials were conducted the average of the results was taken. The average conductivity turned out to be 10.01 microsiemens. Upon completion of the pH test, it was determined that the unknown metal was basic, with the average pH being 8.5. This eliminated any of the compounds with acidic properties. The color of the flame during the flame test was yellowish orange rather than red orange, meaning that the compound received contained sodium as opposed to calcium. During the solubility test, ethanol was chosen to dissolve the compound based on solubility rules, and the compound was insoluble during all three trials, again eliminating the possibilities of sodium nitrate and sodium carbonate. After all tests were performed, the possible identity best suited to the results was sodium acetate Part 2: The three original possibilities were tested again to ensure that the original suspicion of the metal being sodium acetate was correct. Sodium carbonate, acetate, and nitrate were tested for their pH, solubility, and conductivity because their properties were closest to the results obtained in week 1. The pH of all compounds tested was either at a range of 9-11 or neutral, but the one that was closest to the week one results was sodium acetate since the average pH for week 1 was 8.5 and the average pH for week 2 was 9.56. This data caused the elimination of sodium carbonate which has a pH of 10, and sodium nitrate, which has a neutral pH of 7. In the conductivity test, the compound that came closest to the week 1 results was sodium acetate. For the solubility test, we chose to pour silver nitrate in the solution based on the solubility rules. Sodium acetate also had the closest solubility to the week one results, but this time it dissolved. Volumetric analysis, or titration, was chosen as the final step over gravimetric analysis since sodium acetate is a basic salt, and since sodium acetate is base, hydrochloric acid was chosen as the analyte. The final pH of the solution was 6.7, which was right in line with week 1 findings. Despite some skewed results during the titration and quantitative tests, it was still believed that the unknown was sodium acetate. Sources of error/ changes: Systematic Error: During the solubility test, the unknown did not dissolve because there was a small amount of sodium nitrate in the beaker containing the solution. The unknown did dissolve when water was added to the solution, which is why the residue was believed to be present. Also, solubility was also tested using acetone and silver nitrate. A precipitate also formed with these compounds due to the previously stated error. The pH levels for sodium acetate were skewed in week 2 because the solution was more diluted than in the previous week. The color of the solution at the equivalence point during the titration was also affected for this reason. Also, because the pH was not taken at regular intervals during the titration, a data table for titration could not be properly created, and thus no pH curve could be made Random Error: A random error that occurred during this experiment was in the use of the conductivity meter during week 2, where the conductivity values were far from precise. It is difficult to determine whether there was something wrong with the meter, or if it was just human error, but the best guess would probably be human error because the probe was not completely submerged.

Changes/Improvements: One thing that could be done to improve the quality of this experiment if it were to be performed again would be to label the beakers to eliminate the possibility of confusing substances. This occurred several times during the experiment, but which substances were in which beaker was figured out with some thought.

Conclusion The hypothesis for this experiment was that the identity of the unknown metal given was sodium acetate. This was affirmed by our data because sodium acetate has a pH in the range of 8 to 9 as shown in in the pH table for week 1 and the week 2 results, which corresponds with the fact that sodium acetate is a basic salt. The conductivity of the unknown was low as shown by the conductivity test table for week 1, and this makes sense because acetate is a weak electrolyte. The flame in our flame test was a yellow orange color as opposed to orange red, meaning that the metal in the compound was sodium. The compound did not dissolve immediately during the solubility test in week 1 because a small amount of sodium carbonate was present, however it did dissolve in week 2, which again illustrates that speculation were correct because any acetate containing compound is supposed to dissolve without exception. All of the data garnered in the experiment, as shown above, supports the original hypothesis that the metal received was sodium acetate.

Research Connection: In the paper “Investigative proteomics: identification of an unknown plant virus from infected plants using mass spectrometry”. Scientists identified a previously unknown plant virus that was infecting tobacco plants. “Protein extracts were first prepared from leaf tissue of uninfected tobacco plants, and the proteins were visualized with two-dimensional electrophoresis (2-DE). Matching gels were then run using protein extracts of a tobacco plant infected with tobacco mosaic virus (TMV). After visual comparison, the proteins spots that were differentially expressed in infected plant tissues were cut from the gels and analyzed by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Tandem mass spectrometry data of individual peptides was searched with SEQUEST (Cooper, Eckert, Andon, Yates, & Haynes)” The same procedure was used on plants infected with an unknown virus, and the proteins expressed by the HPLC-MS were those of the potato virus X, showing that HPLC-MS is an efficient way of verifying unknown viruses in plants. An article written by Dr. Paul Gates, professor in the School of Chemistry at the University of Bristol, better explains how HPLC-MS works. “As the name suggest the instrumentation comprises a high performance liquid chromatograph (HPLC) attached, via a suitable interface, to a mass spectrometer (MS)...Solutions derived from samples of interest are injected onto an HPLC column that comprises a narrow stainless steel tube (usually 150 mm length and 2 mm internal diameter, or smaller) packed with fine, chemically modified silica particles. Compounds are separated on the basis of their relative interaction with the chemical coating of these particles (stationary phase) and the solvent eluting through the column (mobile phase). Components eluting from the chromatographic column are then introduced to the mass spectrometer via a specialized interface.” (Gates) This experiment is similar to Project 1 in that these scientists prepared solutions and performed qualitative and quantitative analysis on them to verify an unknown. This illustrates

that the process of finding unknowns in useful in many fields of science and can be done in a variety of ways.

References Anderson, L.; Figueroa, J.; Lykourinou, V. General Chemistry I Laboratory Manual, 2nd ed.; University of South Florida, 2016. Cooper, B.; Eckert, D.; Andon, N. L.; Yates, J. R.; Haynes, P. A. Journal of the American Society for Mass Spectrometry 2003, 14 (7), 736–741. Gates, P. High Performance Liquid Chromatography Mass Spectrometry (HPLC/MS) http://www.bris.ac.uk/nerclsmsf/techniques/hplcms.html (accessed Oct 17, 2016)....