Experiment 8 report - Shanice Chevalier Erin Clarke Anna...

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Shanice ChevalierErin ClarkeAnna CipolloneJulia RoubidouxProfessor JessaCHM 144 BNovember 10, 2015Experiment 8- How Much Copper is in a Penny?Introduction To analyze the copper content in a penny the metals must first be dissolved. Pennies are comprised of copper (Cu) and zinc (Zn), which require the use of strong oxidizing agents such as nitric acid. Cu(s)+ 4HNO3(aq) + 2H2O(l) Cu(H2O)42+(aq)+2NO2(g) + 2NO3-(aq)Zn(s)+ 4HNO3(aq) + 2H2O(l)Zn(H2O)42+(aq) + @NO2(g)+2NO3-(aq)This oxidization produces nitrogen dioxide (NO2), which a is a brown toxic gas and the complex ions of Cu (Cu(H2O)42+) and Zn (Zn(H2O)42+). Ligands consist of a central metal ion covalently bonded to two or more anions or molecules. The H2O ligand can be displaced by ammonia (NH3) to create new complex ions, Cu(NH3)42+and Zn(NH3)42+:Cu(H2O)42+(aq) + 4NH3(aq)Cu(NH3)42+(aq) + 4H2O(l)ZN(H2O)42+(aq) + 4NH3(aq)ZN(NH3)42+(aq)+ 4H2O(l)The Cu(NH3)42+ ion has a deep blue color while the ZN(NH3)42+ion is colorless which provides the opportunity to determine the concentration of Cu without disturbance from the larger concentration of Zn. NH3is added to the dissolved penny solution so that the Cu(NH3)42+ complex ion is formed.The main objectives of this experiment are to experimentally determine the amount of copper in a penny using the absorbance characteristics of the complexion Cu(NH3)42+. Absorbance is a measure of the capacity of a sample to absorb
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light and this is done by the use of a UV-visible spectrometer. Therefore another objective is to learn how to use a UV-visible spectrometer, learn how visible and UV absorbance of a compound is solution is related to concentration (Beer-Lambert law). Beer-Lambert law states that the amount of light energy absorbed at a particular wavelength by a compound or solution is proportional to its concentration in that solution. Also, learn about oxidation of less active metals and complex ion formation. The hypothesis for this experiment is that the amount of copper in a penny is no more than 2.5% by mass if it was produced after 1982. This is because due to the increasing price of copper, Congress made a mandate limiting the amount of copper used. This was done 1982; prior to that the penny contained about 95% copper. The pennies used in this experiment we dated, 1995, 1997 and 2011 andtherefore were all subject to the mandate which means they should all consist 2.5% copper by mass and mostly zinc.
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