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
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...