copper-penny-report

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Studocu is not sponsored or endorsed by any college or university Copper Penny report General Chemistry (Cornell University) Studocu is not sponsored or endorsed by any college or university Copper Penny report General Chemistry (Cornell University) Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

Copper Penny Report Natalia Jordan TA: Sewon Oh December 12th, 2020 Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

Ⅰ . Abstract This lab was designed to determine the amount of copper in a post-1982 U.S penny through the process of spectrophotometry. By using the same cuvette, absorbance is directly proportional to concentration, and the graph of those variables demonstrates Beer’s Law. The penny was first dissolved using nitric acid followed by ammonium hydroxide. This newly made copper (II) tetraamine complex is referred to as the “Penny solution” and its absorbance at 620 nm was measured. The absorbance was 0.639. The second part of the experiment was conducted to measure the absorbance of 5 different concentrations of Cu(NH3)4+. A standard curve for the concentration of Cu(NH3)4+ was generated from these values, and the concentration of Cu(NH3)4+ in the “penny solution” was determined to be 0.012978 M using the equation of the trendline from the standard curve graph. The mass of copper was then determined to be Cu from the known molarity and volume of the Cu(NH3)4+ . The percent of error was .08247 g 0 determined by comparing the measured mass of copper in the penny to the actual mass of copper in a post-1982 penny, 0.0625g Cu (found from usmint.gov). The percent error was found to be 31.95%. Ⅱ . Experimental Section Part One: First, a post-1982 penny was obtained and weighed to the nearest milligram. A 25 mL graduated cylinder was used to measure out 15 mL of concentrated nitric acid in the hood. The penny was then laid flat on the bottom of a 200 mL beaker and the 15 mL of nitric acid were slowly added, in the fume hood, on top of the penny. Because all of the copper solutions did not completely dissolve, the contents of the beaker were carefully swirled to ensure that all copper dissolved. The solution initially appeared to be a green/teal color, then became lighter, and then emitted an orange/brown gas. To the beaker, 15 mL of distilled water were added. The contents of the beaker turned from a green color to an aqua blue color. Another beaker was obtained and 25 mL of concentrated ammonium hydroxide was added to the nitric acid solution in portions. The beaker contents were then swiftly swirled after each addition of the ammonium hydroxide. After the first addition of ammonium hydroxide, the solution turned a dark blue and became cloudy after the contents were swirled. In the second addition, the beaker Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

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contents became warm to the touch and a precipitate was visible in the beaker. In the third addition, the solution became a darker blue color. Lastly, in the final addition of ammonium hydroxide, a precipitate no longer remained and the beaker appeared a clear cobalt blue color. The solution was then left alone to cool to room temperature and 100 mL of copper (II) tetra ammonia complex were measured in a volumetric flask and diluted with distilled water to the mark. The solution in the volumetric flask was mixed by inversion several times to ensure it was properly mixed. This solution was labeled the “Penny Solution''. The absorbance of this solution was measured at 620 nm. Part two: Five beakers were thoroughly cleaned and dried. The beakers were labeled 1-5. The first beaker was used to obtain 15 mL of .040 M Cu(NH 3 ) 4 + . A series of 5 serial dilutions was conducted. A volumetric pipette was used to dispense exactly 10.00 mL of distilled water into each of the four beakers labeled 2,3,4, and 5. From beaker 1, a volumetric pipette was used to measure exactly 10.00 mL of the Cu(NH 3 ) 4 + stock solution and it was added into beaker 2. The solution in beaker 2 was swirled to ensure proper mixing. A volumetric pipette was then used to move 10.00 mL of the solution in beaker 2 into beaker 3. The solution in beaker 3 was swirled to ensure proper mixing. A volumetric pipette was then used to move 10.00 mL of the solution in beaker 3 into beaker 4. The solution in beaker 4 was swirled to ensure proper mixing. A volumetric pipette was then used to move 10.00 mL of the solution in beaker 4 into beaker 5. The solution in beaker 5 was swirled to ensure proper mixing. After the 5 serial dilutions were performed, the absorbance of the solution in each of the 5 beakers was determined and recorded at 620 nm. Ⅲ . Results and Discussion: Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

Table 1: Absorbance of Different Solutions measured at 620nm Table 2: Concentration of Cu(NH3)42+ (Part B) in beaker solutions Solution Absorbance (measured at 620nm) Penny Solution 0.639 Beaker 1 2.323 Beaker 2 1.312 Beaker 3 0.592 Beaker 4 0.275 Beaker 5 0.133 Solution Concentration (M) Beaker 1 0.04 Beaker 2 0.02 Beaker 3 0.01 Beaker 4 0.005 Beaker 5 0.0025 Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

Table 3: Absorbance and Concentration of Cu(NH 3 ) 4 2+ (Part B) in beaker solutions Solution Absorbance Concentration (M) Beaker 5 0.133 0.0025 Beaker 4 0.275 0.005 Beaker 3 0.592 0.01 Beaker 2 1.312 0.02 Beaker 1 2.323 0.04 Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

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Graph 1: Absorbance vs. Concentration for Five Beaker Solutions with an equation of A= 53.3x-0.0527 (Table 3 data) During the experiment, wavelength is held constant and the same cuvette is used. Using the same cuvette ensures the path length stays the same, thus Graph 1 illustrates Beer’s Law of A= εdc. Concentration, c, usually has units of M. The distance light travels through the sample, d, usually has units of cm. The molar absorptivity or extinction coefficient, ε, has units of . cm M −1 −1 Therefore, absorbance is unitless. If the same d is used for measurements, absorbance becomes directly proportional to concentration. By using the line of best fit for this graph, it is possible to determine the concentration of in the Penny Solution. In this experiment, u ( NH ) C 3 4 2+ pathlength, d, is 1 and the molar absorptivity, ε, is the slope of the best fit line. Ideally, the y-intercept should be 0, though it appears as -0.0527. Since the Penny Solution’s absorbance is 0.639, its concentration of is as follows: u ( NH ) C 3 4 2+ 3.3 x .0527 A = 5 − 0 .639 3.3 x .0527 0 = 5 − 0 C = 53.3 (0.639+0.0527) .012978 M c = 0 Concentration of in “penny solution” = u ( NH ) C 3 4 2+ .012978 M 0 Where c and x are interchangeable Using this information the grams of copper in the original penny can me calculated: 0.012978 M in 100mL of solution u ( NH ) C 3 4 2+ 00 mL 0012978 mol Cu ( NH ) 1 L 0.012978 mol Cu ( NH ) 3 4 2+ × 1 × 1 L 1000 mL = . 3 4 2+ Since Cu and are in a 1:1 ratio, u ( NH ) C 3 4 2+ Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

= 0012978 mol Cu ( NH ) . 3 4 2+ .0012978 mol Cu 0 Using molar mass of Cu (63.546g): 0012978 mol Cu .08247 g Cu in the original penny . × 1 mol Cu 63.546 Cu = 0 Assuming a penny is a cylinder with a diameter of 19.0mm, height of 1.5mm, and a density of 8.93g/mL (which is equivalent to 8.93g/cm3), the thickness of copper on the penny can be calculated: .08247 g Cu .2351 mm Cu 0 × 1 cm 3 8.93 g Cu × (1 cm ) 3 (10 mm ) 3 = 9 3 surface Area ) thickness ) V Cu = ( penny × ( dh thickness ) V Cu = π × ( .2351 mm (19.0 mm )(1.5 mm ) thickness ) 9 3 = π × ( hickness of copper penny .03 mm T = 1 Assuming that the diameter of a copper atom is , the thickness of copper can be .28 0 mm 2 × 1 −7 calculated to be: .52 0 atoms thick (1.03 mm ) 2.28×10 mm per atom −7 = 4 × 1 5 According to the U.S. Mint, post-1982 pennies are composed of 2.5% Cu by mass. Using the original mass of the penny and this information, the actual mass of copper on a penny can be determined: ctual mass of Cu .500 g (0.025) .0625 g Cu a = 2 = 0 As calculated before, the measured mass of Cu from the experiment is as follows: easured mass of Cu .08247 g Cu M = 0 Thus the percent error is calculated to be: ercent error 00 p = actual value measured value − actual value | | × 1 Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

ercent error 00 1.95 % p = 0.0625 0.08247−0.0625 | | × 1 = 3 Our percent error in determining the mass percent of copper in the penny was 31.10%. We determined that the mass of copper in a penny was 0.08247g when it is really 0.0625 g copper. The sources of error could have possibly arose from some discrepancies in measuring the absorbity for the dilution. The most diluted solution appeared slightly cloudy, and its absorbance was a bit difficult to measure accurately. Likewise, the spectrophotometer may not have been calibrated correctly and the serial dilutions may have been incorrect. These mistakes would cause the standard curve of Cu(NH3)42+ to be incorrect, and lead to a miscalculation of the concentration of copper in the “penny solution,” which would affect the calculation of the copper cladding’s thickness. For example if the spectrophotometer recorded a lower absorbance than it should have, this would lead to a flatter slope of the curver and in return a lower estimated concentration of copper derived from the trendline equation of Cu(NH3)42+. However, since the R2 value, 0.996, is so close to 1, the line of best fit is a good representation of the data. Downloaded by Jeff Tricksy (jefftricksy@gmail.com) lOMoARcPSD|32735920

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