Full version Copper And Zinc Composition Percentages In Pennies

Copper And Zinc Composition Percentages In Pennies

This print version free essay Copper And Zinc Composition Percentages In Pennies.

Category: Miscellaneous

Autor: reviewessays 04 June 2011

Words: 1018 | Pages: 5

Copper and Zinc Composition Percentages in Pennies

Introduction. The United States Mint sends copper and zinc to a fabricator, which creates coin-sized discs called planchets. The planchets undergo the coining press at the Mint where they are stamped as genuine United States legal tender coins. The purpose of this experiment is to determine the accuracy of the copper and zinc composition percentages in a random sampling of pennies. The penny was dissolved to make aqueous copper ions and four copper dilutions were made from stock solution. Each cuvette sample was measured in a colorimeter and the data was plotted linearly using Beer's law plot. Mass percent and percent error were found using calculations. Analysis of class data provided further data to determine the copper and zinc composition percentages.

Experimental Procedure. A penny was weighed on a digital scale. About 15 mL of 10 M HNO3 was measured and placed into medium sized beaker. The beaker was placed under the fume hood and penny was added to the solution. The solution was diluted to 25.00 mL in a 25 mL volumetric flask. The penny solution was put into the flask, covered and mixed to dilute the solution. Filled to line with disposable glass pipette, covered and mixed again. Four dilutions were made from the stock solution with de-ionized water using the concentration calculations in vials. Seven cuvettes were obtained. One cuvette was filled with de-ionized water and one cuvette with penny solution. Five cuvettes were filled with the copper standards. The computer was set up and the absorbance of each of the solutions at 635 nm was measured.

Chemical Equations:

Dilute Acid

8H3O+ (aq) + 2NO3- (aq)  2NO (g) + 12H2O (l)

Strong Acid

4H3O+ (aq) + NO3- (aq)  2NO2 (g) + 6H2O (l)

Zinc Reaction with Acid

2H3O+ (aq)  2H2O (l) + H2 (g)

Formulas Used:

M1V1 = M2V2

Concentration(mol/L) x Volume(L) x Molar mass(g/mol) = Mass (g)

Beer's Law A=Elc+b

Mass Percent: Mass of Component x 100%

Total Mass of Sample

Percent Error: Theoretical Value В– Experimental Value x 100%

Theoretical Value

Results.

Pre-Lab

Amount of stock solution needed for dilutions

(0.050M)(10.00mL)=(0.30M)(x mL)

x = 1.7mL

(0.10M)(10.00mL)=(0.30M)(x mL)

x = 3.3mL

(0.15M)(10.00mL)=(0.30M)(x mL)

x = 5.0mL

(0.20M)(10.00mL)=(0.30M)(x mL)

x = 6.7mL

Penny Mass: 2.5140 g

Concentration mol/L of Copper Transmittance (%T) Absorbance

0.00 99.60 0.002

0.050 75.39 0.123

0.10 51.40 0.289

0.15 29.64 0.528

0.20 25.51 0.593

0.30 17.52 0.756

Table 1. Linear fit data. As the concentration increased the absorbance increased proportionally. The first cuvette contains de-ionized water.

R2=0.9783

Absorbance for Penny Solution: 0.120

Grams of Copper in Penny

Beer's Law

A=Elc+b

0.120 = 2.646C+0.02914

C=0.03434 mol/L

Concentration(mol/L) x Volume(L) x Molar mass(g/mol) = Mass (g)

0.03434 mol/L x 0.025 L x 63.55 g/mol = 0.0545 g

Mass Percent of Copper

0.0545g x 100 = 2.17%

2.5140g

Percent Error of Copper

2.17g - 2.5140g x 100 = 13.76%

2.51540g

Group Mass Percent Percent Error

1 2.167% 13.76%

2 2.63% 5.20%

3 3.11% 24.4%

4 3.16% 26.4%

5 4.40% 76.0%

6 3.53% 41.2%

7 3.5% 40.0%

Table 2. Class data was reported for mass percent and percent error of copper composition in the penny. The majority of the mass percent was above the appropriate copper mass percent.

Discussion. Based on the mass percent of copper in the samplings of pennies, the fabricator is not putting exactly 2.5% of copper into the planchets. The results could be caused by various sources of error such as inaccurate measurements of the solutions, contamination of the penny, and improper cleaning of cuvettes. According to the procedures, measurements of nitric acid, dilutions of copper concentrations, and stock solutions all had to be completed. Any deviation from the appropriate amount could affect the cuvette sample concentration and absorbance. This in turn would affect the linear plot and correlation between the points. Because the class used slightly different amounts of the solutions, this could have attributed to the varying class data and sometimes high percent error. The penny could have been contaminated by handling or corrosion over time which affects the composition of the penny including the fact that the penny could have been produced up to twenty years ago. The copper on the penny is located mainly on the exterior so it would be more affected by corrosion than the zinc. Improvements to the experiment would be to use pennies that have been recently manufactured by the Mint in the experiment. The pennies would not be altered as much by external factors that could change the composition. The cuvettes had some minor scratches, discoloration, or fingerprints that could affect the transmission of light through the solution by the colorimeter. Some of the light could have been deflected off. In addition, when measuring the de-ionized water sample, the transmittance was under 100% and the absorbance was higher than zero. By using new and clean cuvettes, there would be less chance for contamination of chemicals, discoloration or fingerprints.

The percent mass of copper in the penny of 2.167% was less than the actual percent mass of 2.5%. This was the only sample where the percent mass was a smaller amount, and all other class data was slightly higher than the actual percent mass. Therefore, the fabricator is most likely using less zinc and more copper than specified. The graph is useful because the pattern of the line and the equation of Beer's law can be used to determine the molarity (concentration) of a copper solution by putting in the absorbance of the penny solution and solving for the unknown. The correlation is a measure of how well the points correlate to the best-fit line. Using the correlation (R2) value of the graph can help determine the amount of error in the experiment. The R2 value which should be as close to one as possible was 0.9783 in this experiment meaning error was present.

In conclusion, this experiment shows that the percentage of copper in pennies is not exactly 2.5%, but this could be caused by environmental factors over time. The average percent error of the class data is not high enough to be of significance. By improving this experiment, closer percentages will likely be obtained. Beer's law is an effective way to find the concentration of copper in the penny as long as the correlation is one.

Further questions raised by this experiment:

What was the copper/zinc percent composition before 1985?

How does the zinc in the penny affect the penny solution?