Experiment 3
The Stoichiometry and Reactions of Some Copper Compounds

POSTLAB QUESTIONS

1. What is meant by "digestion of a precipitate"?

2. A student makes the following mistakes in the experiment you have just completed. For each of the following scenarios, indicate whether the student's error would be expected to cause an increase, decrease or no change in the apparent mass of copper recovered at the end of the experiment. Give a brief explanation for your response.

a.	The NaOH is added so rapidly that a blue gelatinous precipitate is not observed and the solution turns immediately to a black precipitate (Steps II and III).
b.	The student does not wait until fizzing has stopped in Step V before decanting and washing with water and acetone.
c.	The student does not carefully wash the copper recovered in Step V with water before washing with acetone and drying.
d.	All the copper is not dissolved in HNO3 in Step I.
e.	The black precipitate in Step III is not allowed to settle before decanting.
f.	When the student tests the solution in Step V with NH3 the mixture turns blue. The student immediately decants and adds HCl.

3. The copper recovered at the end of the experiment may not appear to be shiny. It may, in fact, appear as a dark precipitate. Why?

4. Explain the difference between the terms "clear" and "colorless" (i.e. strained apple juice is brown and clear; water is colorless).