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Percent Mass of Copper in a Penny

By R3volt Nov 05, 2008 1165 Words
Percent Mass of Copper in a Penny
Introduction
The purpose of this lab is to determine the percent mass of Cu in a penny and see if the fabricator that makes the planchets for the government is using the correct amounts of Cu in the pennies. The composition of a standard penny is 97.5% Zn and 2.5% Cu. In the lab we will be using Beer’s Law (A=elc+b where A is solution absorbance, e is a constant called molar absorbency, l is the length in cm, and c is the concentration). Using Beer’s Law in this lab a colorimeter is used to find the absorbance and from this the concentration of dissolved Cu2+ ions can be found and percent mass calculated. The techniques used in this lab are useful in that they provide little human error for various parts of the lab by taking the measurements by a colorimeter human error is reduced. Experimental Procedure

Equations Used
M1V1=M2V2, Beers Law: A=elc+b, Morality= M = (mols of solute)/ (Liters of Solution) Reaction: 3Cu(s) + 8 H3O+ (aq) + 2NO3- (aq)  3Cu2+(aq) + 2NO (g) + 12 H2O (l)
Cu(s) + 4 H3O+ (aq) + 2NO3- (aq)  Cu2+(aq) + 2NO2 (g) + 6 H2O (l)
Zn (s) + 2H3O+ (aq)  Zn2+ (aq) + 2H2O(l) + H2 (g)
First weigh a post 1982 penny. Measure about 15mL of 10M HNO3 and put in medium sized beaker under the fume hood adding the penny to the solution to be dissolved. Dilute your solution to 25.0mL in a 25.0mL volumetric flask. Make four dilutions from the 25mL stock solution, using deionized water. Store each solution in a labeled vial or test tube. Prepare 5 cuvettes, one with the stock solution and 4 from the previous dilutions. Fill one cuvette with deionized water and another with your penny solution. Use a different dropper for each transfer. Set colorimeter to 635 nm and insert the cuvette filled with deionized water into colorimeter. Close colorimeter and hold the cal button to calibrate the colorimeter. Record your data. Whenever ready to start press Collect and place the first solution into the colorimeter (rough sides facing left and right). Press Keep and enter concentration of solution to plot a point on the graph. Repeat for all cuvettes of standard solutions. Do the same for the penny solution and record the green absorbance in your lab book.

Data and Observations
Absorbance of Cu solutions:
Wave Length = 635 nm
Concentration (M)Absorbance
0.0500.101
0.1000.337
0.1500.413
0.2000.548
0.2990.746
Calibration Curve: y = 2.46X +0.0354
Absorbance of Penny Solution = 0.217

Class Data with %Mass and % error of Cu in pennies
Group #% Mass% Error
14.7%88%
23.6%44%
33.9%24.8%
44.4%39.9%
55.4%116%
61.8%27.5%
74.8%92%

The reaction with the penny and Nitric acid in order to dissolve the penny was a very colorful reaction. At first the solution bubbled and then an orange-brown gas was released and the final solution was Blue – Green. In table one the concentration transmittance and absorbance are show witch give the point to the calibration curve used below. The 0.9993 Correlation demonstrates how the relationship between concentration and absorbance are directly related as one increases so does the other. The % mass of Cu in an actual post-1982 penny is 2.5% mass. This shows that the class discovered higher percent mass calculation than actually in the penny yet the data is close enough to verify that the % mass of Cu in the pennies is 2.5%. The percent errors for this lab as a whole varied significantly which shows that the procedures in the lab could have problems. This may be caused by the significant reliance on human measurement.

Discussion
In this lab a dissolved penny to produced Cu2+ ions in a solution to measure absorbance in a colorimeter and determine the concentration of the Cu2+ ions to discover the % mass of Cu in a penny. A calibration curve was formed for this lab to show the relationship between absorbance of Cu2+ ions and concentration so that the concentration of Cu2+ ions in the penny solution can be found. The correlation for the lab data of .9993 demonstrates that the data found in this lab was not due to chance and that concentration and absorbance have an actual positive relationship meaning that as concentration of Cu2+ ions increase so does the absorbance.

Some sources of possible error for this lab can come from human error in measurements. When diluting the stock solutions using the pipette to transfer the correct amount of stock solution was not very precise or accurate. The tick marks of the pipette weren’t small enough for the scale of this lab and would have been more useful if it was divided into smaller units. The skill of the reader also comes into play here for an unskilled reader will not be quick enough to properly stop the pipette from putting in too much of the stock solution. This will result in the appearance of a higher Cu2+ ion concentration of a penny. Also contamination may occur if the equipment used was not properly cleaned. This will result in a higher absorbance and thus showing that the Cu2+ ions have a higher concentration.

Some improvements in order to minimize error in this lab will be found in the minimizing the human error for this lab. If instead of having the human dilute the solutions, automate the process and have a computer do it exactly so that the proper calibration curve can be found. This will lead to smaller error by having a more precise concentration. Also automate the transportation process of the solutions. This will prevent any spillage of solution and thus no Cu2+ ions will be lost or spilt to give false values. The more the human error is taken out of the process the more precise and accurate the results will be.

The purpose of this lab is to find the percent mass of Cu in a post 1982 penny. This was found by forming a calibration curve based on a stock solution so that the concentration of the ions in the dissolved penny could be discovered and from that percent mass found. The procedures for this lab are good but the equipment such as the pipette posed problems and lead to inaccurate readings and a larger percent error. Overall the procedures and using of the colorimeter where very helpful in minimizing the percent error and taking the human error out of the equation for this lab.

Further Questions
Why did everyone get a higher percent mass than there actually was shouldn’t there be some groups who get lower? How different would our results be if we used a penny that was pre-1982?

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