AP Lab 7: Empirical Formula of Copper Iodide
Introduction:
The empirical formula of a compound describes the simplest whole number ratio of a compound. While it
may not represent the actual ratio in the molecular formula, the empirical formula can be extremely useful in
the analysis of various chemical compounds.
In this lab, you and a partner will be determining the empirical formula of the copper iodide compound
produced by direct synthesis from the elements, for the oxidation-reduction reaction:
Cu(s) + I2(g) → CuxIy(s)
Because copper is capable of taking on numerous oxidation states, it is important to know the empirical
formula so that the oxidation state of the compound can be determined. By knowing the amount of copper
that was started with and the mass of the final copper iodide product, the amount of copper and iodine
reacted can be determined and, therefore, the percent of each in the copper iodine compound can be
determined.
Procedure:
Pre-lab:
1. Make the following chart in your lab notebook:
Mass of Cu Strip
Mass of Cu + I
Mass of I
Reacted
Mass of Cu Strip
(same as Mass of
Cu for next trial)
Mass of Cu
Reacted
Trial 1
Trial 2
Trial 3
2. The initial mass of copper that you began with was 5.5 g and the final percents you determine are
20.03% Cu and 79.97% I. What is the empirical formula of this compound?
Lab:
NOTE: During the entire lab, only pick up the copper strip by the edges. You want a little finger prints on the
copper as possible.
CAUTION: Iodine gas is very toxic. Only four people max in the back room at one time. Only one person
per group should take the copper to the fume hood at a time.
1. Clean a strip of copper by dipping it in water and then in acetone. Then shake dry.
2. Weigh the Cu strip to the nearest 0.001 g.
3. One person in your group, bend the copper strip in half at a 90° angle and hang it over the edge of the
beaker holding the iodine in the fume hood.
4. Leave the copper in the iodine for 90 seconds.
5. Remove your copper strip from the iodine. Run it through the alcohol burner flame until there is not
black/blue iodine on the copper strip. There should only be a light coating of copper iodide which is
a whitish/yellow/tan color.
6. Let the strip cool for 30 seconds.
7. Weigh the copper strip with the CuxIy crystals.
8. Dip the copper strip into the solution of sodium thiosulfate, swirl it around in solution to remove the
copper iodide crystals.
9. Repeat steps 1-8 two more times.
10. Repeat step 1 again and return the clean dry strip of copper to the front table.
LABS
Results/Discussion:
Analysis:
1. Calculate the mass of iodine and copper used in each trial and fill them into your table. Be sure to
show all of your work in your lab notebook.
2. Make all the necessary calculations to determine the empirical formula of copper iodine, CuxIy.
Show all the setup, units and calculations in your lab notebook. Report the empirical
formula on the whiteboard at the front of the classroom.
Conclusion Questions:
1. What did you find was the empirical formula for the copper iodide compound produced? Based
on this formula, what is the charge on the copper in the compound? Give the proper name
for this compound.
Give one error that could have affected your data and explain how that factor might have changed
your data.
2. Explain how each of the following would affect the empirical formula that could be calculated:
a. Some unreacted iodine condensed on the strip.
b. A small amount of the white compound flaked off before weighing.
LABS