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Elizabeth GaffneyChemistry 144ANefertiti MuhammadMargaret F.11/11/13Finding out the amount of Copper in a pennyIntroduction:
To find out how much Copper was in a given penny there was some background information that must have been known first. A strong oxidizing agent is required to oxidize copper. Nitric Acid is a strong Oxidizing agent that could dissolve copper. WhenNitric acid is dissolved in the metals that make up a penny it creates nitrogen dioxide, a brown toxic gas and the complex ions (Cu(H2O)42+) and (Zn(H2O)42+). Copper with NitricAcid creates a deep-blue color and Zinc with Nitric Acid is colorless. The Beer-Lambert law helps to understand the absorbance with molecular species and the absorbance with copper. According to it, absorbance is directly proportional to the concentration of the absorbing species in the solution. The main objectives for the experiment included to figure out the absorbance of the copper and nitric acid complex ion and with that determine the amount of copper in a penny. Another objective is to learn more about the oxidation with complex ion formation and less active metals. To learn how to prepare a calibration curve based on the Beer-Lambert law and to learn how to use a UV-visible spectrometer. The last objective for the experiment was to learn how visible and UV absorbance of a compound in solution is related to its concentration. The hypothesis for the lab was that there was 2.5% of copper in a penny. This was known because that is how much copper is authorized by congress to be used in a penny. In the late 1800s the penny contained 95% copper but since the cost of copper became higher than what the penny was worth. Now there is more zinc metal in pennies to keep the amount and cost of copper down. Since most of the copper is on the outside and the three pennies that were given in the experiment were all worn and hard some dirt on them it could be possible that there is less copper on the outside of the penny than 2.5%. Methods:Solution preparationThe standard solutions were prepared by first pipetting 0.0 mL, 2.00 mL, 4.00 mL, 6.00 mL, 8.00 mL, and 10.00 mL of 5.00 g/L stock copper solution. Three mL of concentrated 15 M ammonia solution was then added to each of the pipetted solutions in a 50 mL volumetric flask. Once all of the solids were dissolved, water was added to each2
of the solutions to dilute them. When the dilutions were complete the concentration for copper for each of the six solutions were calculated by taking the amount of stock