0.7 Colorful copper  (Page 2/2)

The percent yield of the copper can be expressed as the ratio of the recovered mass to initial mass, multiplied by 100:

% yield = (recovered mass of Cu/initial mass of Cu) x 100

Experimental procedure

SAFETY PRECAUTIONS: Wear safety glasses and gloves when handling acids. Work in the fume hood. Acetone and Methanol are Flammable, so keep them away from flames. Avoid breathing any vapors, especially methanol, as it is very toxic.

1. Preheat a hot plate in the fume hood.

2. Place one strip of fine copper wire (Remember to record the actual weight, approximately 0.5 g) in a 250 mL beaker. Bend the copper wire so that it rests on the bottom of the beaker.

3. Slowly add 30 mL of 6 M ${\text{HNO}}_3$ . Perform this step in the fume hood, as the gas produced is toxic. You may need to gently heat the solution. Observe the color of the gas and solution. Place a wet paper towel and watchglass over the beaker to dissolve the gas. Ask your TA if you are unsure how to do this.

4. Wait until the copper wire dissolves completely before proceeding.

5. Remove beaker from hot plate with a hot hand. Remove watchglass and paper towel. Be sure that the hood is closed as much as possible at this point to avoid breathing in the gas.

6. Add 100 mL of deionized water (Do not use regular water!)

7. Slowly add 30 mL of 6 M NaOH. Note the color of the precipitate and evolution of heat.

8. Add 2 or 3 boiling chips and carefully heat the solution just to boiling point. Note any color change. While waiting for this solution to heat, begin heating ~200 mL deionized water (in a 400 mL beaker) for the next step.

9. Decant some of the supernatant liquid into a 500 mL beaker (Try not to lose any solid while decanting. It is ok to leave some of the liquid behind.). Add about 200 mL of very hot deionized water and allow the precipitate to settle. Decant once more. What are you removing by washing and decantation?

10. Heat the beaker containing the solid for 20 minutes to reduce the volume of the solution (or until the volume has been reduced by half). Having a more concentrated solution will make the following steps proceed faster.

11. Add 15 mL of 6 M $H_2{\text{SO}}_4$ to the black tarry substance in the beaker (not the solution that you have decanted) while stirring with a glass rod. What copper compound is present now?

12. Remove the boiling chips.

13. In the hood, add 5-10 one-inch squares of aluminum foil and 5-10 ml of concentrated HCl, noting any color changes.STIR WELL. An ideal ratio should be 7 pieces of Aluminum to 10 ml of acid but you might need more or less depending on the success of your previous steps. If your solution turns green stop and ask your TA for help. Otherwise, continue to add pieces of aluminum until the supernatant is not blue. Identify what forms on the surface of the aluminum. What is present in the solution? What gas is formed in the reaction? How do you know?

14. When gas evolution has ceased, decant the solution and transfer the precipitate to a preweighed 100 mL beaker and record its mass on your report form. Wash the precipitated copper with about 5 mL of distilled water and allow it to settle before you decant the solution, and repeat the process. Then wash the precipitate with about 5 mL of methanol. Allow the precipitate to settle, and then decant the methanol. Finally, wash the precipitate with about 5 mL of acetone. Allow the precipitate to settle again and then decant the acetone from the precipitate. What are you removing by washing?

15. Then use the microwaves to dry your product. Start with 30 seconds and then try 10 seconds more until it is dry. Be sure to let your product cool before weighing it. Then calculate the final mass of copper and from there the weight and percent yield can be determined. Compare the mass with your initial mass and calculate the percent yield. What color is your copper sample in the final step?

Is it uniform in appearance?

Suggest possible sources of error in this experiment.