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Determination of % Composition of Pennies Using Oxidation-Reduction and Double Displacement (Precipitation) Reactions

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:

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1. What is the weight of a post 1982 penny?

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

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

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

5. Write a balanced reaction of zinc with HCl.

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

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?

8. 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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