Exp 7 Lab report revised - Experiment 7 How Much Copper in...

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Experiment 7 How Much Copper in a Penny? Alexx Sikora Lab Partner(s): Danielle Moroski, Hannah Vitiello, Claire Turner Dr. Zhu TA: Dhilhani CHM 144M/A 2 November, 2016 1
Abstract The purpose of this experiment was to test a penny, after 1982, to see the percentage of copper used to make them. This was done by preparing six standard solutions of Cu 2+ . The penny was then placed in 6M HNO 3, to find the percent of copper it was dissolved by adding concentrated ammonia to the solution. This mixture produced a complex copper ion, which was then used throughout the lab. A spectrophotometer was used to detect the wavelengths of the six standard solutions. One standard solution was filled with distilled water (the blank). The blank was placed in the “R” slot, while the other five solutions were placed in the “S” slot of the spectrophotometer. A calibration curve was created and the equation of this was used for further calculations. An extrapolated Interpol value was also used for calculations, found from the calibration curve equation. The hypothesis tested stemmed from the proposal by U.S Congress, that a penny should contain 2.5% copper. However, the results from the standard deviation and T-test concluded that the pennies are not within a 95% confidence level to the regulated 2.5%. Introduction For this experiment to take place, students needed to understand redox reactions and the oxidative reaction between that of copper (in the penny) and nitric acid. The nitric acid took place as the oxidizing agent of copper. As the penny was dissolved, the reaction released nitrogen dioxide, a gas, and complex ions. These ions being 6H 2 O, copper, and 2NO 3 - , making the solution a blue color. The ions were isolated by the addition of concentrated ammonia, causing a reaction, resulting with the equation [Cu(NH 3)4 (H 2 O) 2 ] 2+ . Throughout the experiment the students would analyze three different reactions, listed below. Cu (s) +4HNO 3(aq) +4H 2 O (l) Cu(H 2 O) 6 2+ (aq) + 2NO 2(g) +2NO 3 - (aq) Zn (s) +4HNO 3(aq) +4H 2 O (l) Zn (H 2 O) 6 2+ (aq) + 2NO 2(g) +2NO 3 - (aq) Cu(H 2 O) 6 2+ (aq) +4NH 3(aq) [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) +4H 2 O (l) Zn(H 2 O) 6 2+ (aq) +4NH 3(aq) [Zn(NH 3 ) 4 (H 2 O) 2 ] 2+ (aq) +4H 2 O (l) Given this information, students were able to focus on the following purposes. To understand how absorbance of light by a compound in a solution is related to its concentration, with the understanding of the Beer-Lambert Law, by spectrophotometry. Also, to learn about oxidation in a redox reaction producing complex ions. Finally, to determine experimentally the amount of copper in a penny by using the absorbance characteristic of complex ion Cu(NH 3 ) 4 2+ .

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