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 also requires one to read a spectrophotometer correctly, conduct and interpret calibration curves to 1
analyze data, and how to 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. Methods Standard Solution Preparation 5.00g/L stock Cu2+ solution will be given, use a clean beaker to obtain about 40 mL. Using a pipet and six 50 mL volumetric flasks, the solutions will be separated with 0.00 mL, 2.00 mL, 4.00 mL, 6.00 mL, 8.00 mL, and 10.00 mL of the stock Cu2+ solution. The 0.00 mL will correspond with the blank solution for the spectrometric analysis. Each volumetric flask will then contain 3mL of 15 M of concentrated ammonia and will dissolve upon swirling. Dilute each with distilled water and cover with Parafilm to achieve appropriate levels of mixing. The concentration will then be calculated for the six solutions using the formula M1V1=M2V2 with the concentration of the stock solution being 5.00 g/L. Penny Preparation Three pennies will be given to each member, where the weight, age, and condition will be recorded. In the fume hood, place the penny in the 150 mL beaker and add 25 mL of 6 M HNO3 and watch the entire penny dissolve and cool for 10 minutes. Quantitatively transfer all contents into a 100 mL beaker to ensure all contents are obtained. This is done with 5-10 mL of distilled water three times. Then add 14 mL of the concentrated ammonia to the flask using a graduated cylinder and observe the blue color from Cu(NH3)4)2+ and watch it dissolve the solid material. Then dilute with distilled water and mix. One cuvette for each penny will be used and filled about ¾ full. Measurement of Absorbance The absorbance value should rest between the highest and lowest absorbance values for the standard solutions from a spectrometer. The maximum wavelength is determined by its “highest peak” and due to the blue color of the solution, it corresponds to the blue, green color reflected. The range of the spectrum should be set between 400 to 700 nm taking into account the 200-350nm is from the absorbance of light from the plastic cuvette. The most and least concentrated solution are used (0.00 and 1.00 g/L) and rinsed and diluted. This procedure is repeated with all the other dilutions along
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