Copper and Zinc Composition Percentages in Pennies

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 Ð'- 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