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Determination of % Composition of Pennies Using Redox and Double Displacement (Precipitation) Reactions Essay

Introduction:
Oxidation involves the gain of electrons of hydrogen or the loss of oxygen or decrease in oxidation state. If zinc completely reacts with HCL, then the theoretical yield of copper should be equivalent to the actual yield.

Purpose:

In this lab, we will determine the percent composition of a modern (post-1982) penny by using a strong acid to react and dissolve the zinc core, leaving only the copper coating. Once only copper remains, we will compare its mass to the entire mass of the penny to determine how much of a penny is copper and how much is zinc. 1. Obtain one 50.0 mL beaker, and label the beaker with your NAME and HOUR.

2. Obtain a penny dated 1982 or later. Wash the penny with soap and water to get it clean.

3. Using a triangular file to make FOUR oppositely-placed small grooves into the edge of the penny (the marks with be approximately 90 degrees apart. The grooves must be deep enough so that the zinc is exposed, but not so deep that the penny’s mass is greatly affected.

4. Clean the top and bottom surface of the penny with steel wool until it’s shiny. Rinse the penny in acetone and dry it on paper towel.

5. Determine the mass of the penny on the balance, and record the mass in the table on the bottom of the page. Removed the penny from the balance using tweezers and place it in its beaker.

6. While wearing gloves, carefully pour 50 mL 3M HCl into the beaker.

7. Place your labeled beaker under the fume hood to react. Observe the effect the acid has on the copper outside of the penny.

Questions to answer to help guide the development of procedures for the determination of the percent copper and zinc in pennies through titration and gravimetric techniques:
1. What is the weight of a post 1982 penny?

2.5 grams

2. What is the percent copper and zinc in a post 1982 penny?

97.5% zinc, 2.5% copper

3. How many grams of copper and zinc are in a post 1982 penny?
97.5 grams zinc, 2.5 grams copper

4. How many moles of copper and zinc are in post 1982 pennies?

5. Write a balanced reaction of zinc with HCl.

Zn(s) + 2HCl(aq) –> ZnCl2(aq) + H2(g).

6. How many moles of HCl are needed to react completely with all of the zinc in a post 1982 penny?

2.5

7. In a procedure developed to determine the percent zinc in post 1982 pennies, 50 ml of an HCl solution was used to react (dissolve) all of the zinc in the penny. To ensure complete reaction, the solution contains twice as many moles of HCl that is actually needed. What concentration of HCl should be used?

In the scenario described in problem 7, what is the amount (in moles) of
excess (unreacted) HCl in solution?

9. How many moles of NaOH would be needed to completely react with all of the excess HCl determined in problem 8?
10. As described in problem 7, a procedure was developed to determine the percent zinc in post 1982 pennies. In that procedure 50 ml of an HCl was used to react (dissolve) all of the zinc in the penny. To ensure complete reaction, the solution contains twice as many moles of HCl that is actually needed. To determine the percent zinc in the penny, the excess (unreacted) HCl was titrated with NaOH. Determine the concentration of NaOH needed if you want to use approximately 25 mL of NaOH to titrate the excess HCl.

11. Write the balanced chemical reaction of zinc with HCl (same as problem 5). Is the product of this reaction soluble in aqueous solution?
12. Write the balanced chemical reaction of the product of the reaction described above (problem 11) with NaOH. Is the product of this reaction soluble in aqueous solution?


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