Hannah Merkle Josie and Ace TA Jess Bases, Professor Jessa CHM144 A 11/9/15 Percentage of Copper in Pennies Introduction The conduction of this experiment will provide one with the information to determine the amount of copper present in a standard penny by using two other penny samples from team members to obtain an average. This lab requires one to read a spectrophotometer correctly, conduct and interpret calibration curves to analyze data, and use the absorbance reading to find the concentration of copper from the absorbance using the Beer-Lambert Law. One prediction is that the mass of copper present in the penny samples will be close to the amount of 2.5% authorized by Congress after 1982 since all the penny samples were all created after. A quantitative analysis of a penny will be performed to determine how much copper is actually in a penny, the more deep blue the substance, the more copper is present in that given sample. The zinc that is present in the penny will display a colorless solution therefore we cannot use the spectrometer. An oxidation- reduction reaction occurred between the copper and the zinc and was later dissolved using a stronger acid called nitric acid, which is also a powerful oxidizing agent and will result in a brown toxic gas called nitrogen dioxide. Aqueous solutions of nitric acid are used in this experiment because it has the ability to dissolve or oxidize copper. With the addition of Nitric acid (HNO3), the copper will be reduced and will eventually produce a complex ion of (Cu(H20)6 2+) and (Zn(H20)6 2+). A central ion has a number of other molecules or ions surrounding it called ligands. The ligands always have a pair of non-bonded electrons and in this case the six H20 ligands are covalently bonded to the central metal
ion of Cu+2 and can also be displaced by ammonia to generate two different complex ions, which will be added in order to dissolve the penny and so the complex ion of (Cu(NH3)4 (H202)2+) will be formed. The concentration of copper will be determined by the deep blue color ion, and a transparent or colorless solution will appear from the zinc ion. The difference in colors will give one the information to determine the amount of copper present in the penny without destructing the colorless zinc ion that is formed after the nitric acid is added. Cuvettes are used to place the solutions of the penny into and will be compared to the stock solution in order to determine the percent absorbance by comparing the copper in the solution to the pure copper solution, which is a deep blue color. A spectrometer will be used to determine the concentrations of the penny solutions
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