Percent of Cu
in a post 1982 cent
What is the experimental % of copper (Cu) in a post 1982 cent? In this lab, we will determine the % composition of a modern (post-1982) penny by using a strong acid called nitric acid (HNO3), to react and dissolve the zinc core, leaving only the copper coating. Once only copper remains, we will then compare its’ absorbency to the other five post-1982 penny Cu concentrations made in this lab.
Before 1982 the US penny was made of pure copper. Because of the high cost of copper, the US government changed the composition of the penny. As of 1982, pennies have been made of mostly zinc, with a thin copper layer on the outside. The first step in finding the % of copper is to make solution out of the penny. In other words “liquefy” the penny. To differentiate between the copper and zinc; zinc is clear and copper turns blue. During this lab, we will be making several standard solutions of this penny solution. To find out each solutions concentration, we will find its molarity by using the formula:
Cc x Vc = CD x VD
Beer's Law states that the quantity of light absorbed by a substance dissolved in a non-absorbing solvent is directly proportional to the concentration of the substance and the path length of the light through the solution. In order to find the solutions absorbency, we must first use a spectrophotometer which is a machine for determining the ability of a solution to absorb light of a specific wavelength by measuring the transmitted light. This must be done first in order to find a solution’s absorbency. By using the standard solutions previously made, the absorbency of each concentration solution will help find the pennies % of copper in your post-1982 penny.
Equation for the reactions between the post-1982 penny and nitric acid: Blue color
Cu(s) + HNO3(aq) → Cu(NO3)2(aq) + NO2(g) + 2H2O(l)
Zn(s) + HNO3(aq) → Zn(NO3)2(aq) + NO2(g) + 2H2O(l)
Reaction with ammonia added to copper and zinc solutions:
Cu(H2O)62+(aq) + 4NH3(aq) → Cu(NH3)42+(aq) + 6H2O(l)
Zn(H2O)62+(aq) + 4NH3(aq) → Zn(H2O)62+(aq) + 4NH3(aq)
FOR STATIONS 2-5 MAKING STARDARD SOLUTIONS AND THE SPECTROPHOTOMETER:
1) Make a stock solution about 25mL of Cu(NO3)2. This solution will also be used to make the other solutions for this lab. When making solution do not forget to add the ammonia. This will give the solution its deep blue color.
Clean a 1.0 mL pipette with distilled water twice. Put aside after cleaned.
You will then use a flask with your 0.0250 M Cu(NO3)2 stock solution, and using the pipette to suck up all the stock solution just made.
2) Clean the 25 mL flask with distilled water
In a 5.0 mL flask, put in ammonia 6M NH3 using a graduated cylinder
Add in the 25 mL volumetric flask, 0.0250M Cu(NO3)2 stock solution, using a 1 mL pipette.
Filling the flask with distilled water, you will then place the new solution in a 50 mL beaker. Make sure beaker is clean and dry!
Label and cover beaker
3) Repeat set 1 and 2 but using a 2.0 mL pipette. Then use 3, 4 ,5 mL pipettes in the following 4 new solutions. All together you should have 5 Cu(NO3)2 solutions all at different concentrations. 4) Find molarity of each solution with the given equation:
Cc x Vc = CD x VD
5) Fill 7 test tubes with the 5 new solutions with their molarity written in the test tube. Make sure all test tubes are clean and dry!
Note** Each test tube should be filled within 1 cm from the top
6) With one of the test tubes fill it with the penny solution made by STATION 1 in your class. 7) Then with the last and final test tube, fill it with distilled water only
**ALL TOGETHER YOU NOW SHOULD HAVE 7 TEST TUBES FILLED, READY FOR TRANSMITTANCE READING BY THE SPECTROPHOTOMETER**
8) Finally, carefully read calibration instruction given on...
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