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Copper Penny Report Natalia Jordan TA: Sewon Oh December 12th, 2020

. 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 0.08247gCu from the known molarity and volume of the Cu(NH3)4+ . The percent of error was 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 32.40%.

. Experimental Section Part One: First, a post-1982 penny was obtained and weighed to the nearest milligram, it was found to be 2.491g . 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 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.

. Results and Discussion:

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

Table 1: Absorbance of Different Solutions measured at 620nm

Solution

Concentration (M)

Beaker 1

0.04

Beaker 2

0.02

Beaker 3

0.01

Beaker 4

0.005

Beaker 5

0.0025

Table 2: Concentration of Cu(NH3)42+ (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

 (Part B) in beaker solutions Table 3: Absorbance and Concentration of Cu(NH 3 ) 4 2+ 

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 M −1 cm−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 C u(N H 3 )4

2+

in the Penny Solution. In this experiment,

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 C u(N H 3 )4

2+

is as follows:

A = 53.3x − 0.0527 0.639 = 53.3x − 0.0527 C=

(0.639+0.0527) 53.3

c = 0.012978 M Concentration of C u(N H 3 )4

2+

in “penny solution” = 0.012978 M

Where c and x are interchangeable Using this information the grams of copper in the original penny can me calculated: 0.012978 M C u(N H 3 )4 0.012978 mol Cu(NH3 )4 1L

2+

× 100 mL ×

2+

in 100mL of solution

1L 1000 mL

Since Cu and C u(N H 3 )4

2+

= .0012978 mol C u(N H 3) 4 are in a 1:1 ratio,

2+

.0012978 mol C u(N H 3 )4 2+ = 0.0012978 mol C u Using molar mass of Cu (63.546g): . 0012978 mol C u ×

63.546 Cu 1 mol Cu

= 0.08247g Cu in the original penny

Mass percent of copper in the penny: mass Cu in penny total mass of penny 0.08247g 2.491g

× 100

= 3.3107% Cu in the penny

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: 0.08247g Cu ×

1 cm3 8.93g Cu

3

×

(10 mm) (1cm)3

= 9.2351 mm3 Cu

V Cu = (surf ace Areapenny ) × (thickness) V Cu = π dh × (thickness) 9.2351 mm3 = π(19.0mm)(1.5mm) × (thickness) T hickness of copper penny = 1.03 mm Assuming that the diameter of a copper atom is 2.28 × 10−7 mm , the thickness of copper can be calculated to be: (1.03 mm) 2.28×10−7 mm per atom

= 4.52 × 105atoms thick

According to the U.S. Mint, post-1982 pennies are composed of 2.5% Cu by mass. Using the this, the percent error is calculated to be: value−actual value| percent error = | measuredactual × 1 00 value

percent error =

| 3.31−2.5| 2.5

× 100 = 32.40 %

Our percent error in determining the mass percent of copper in the penny was 32.40%. We determined that the percent mass of copper in a penny was 3.31 % when in reality it is 2.5% 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....