Name: ___________________________________________________ Date: _________
Empirical Formula of a Copper
Compound
Purpose
To determine the empirical formula of a sample of copper chloride.
Introduction
The empirical formula of a compound tells us the types of atoms present
in a compound as well as the simplest whole-number ratio of the
different types of atoms. The empirical formula does not tell us the actual
number of atoms in the molecule. For example, the molecular formula of
the compound butane is C4H10. This formula tells us that one molecule
of butane contains four carbon atoms and ten hydrogen atoms. The
empirical formula of butane, on the other hand, is C2H5, since the
simplest whole-number ratio of carbon to hydrogen atoms in the
compound is 2 to 5.
An ionic compound is three-dimensional array of ions, which are present
in a definite ionic ratio that is characteristic of the compound. Since
there are no discrete molecules in the lattice of an ionic substance, only
an empirical formula can be used to express its composition. Thus, for
magnesium chloride, which is composed of a collection of Mg2+ and Fions in the ratio of one to two, the empirical formula is MgF2. There is no
molecular formula for magnesium chloride.
In this experiment you will determine the empirical formula of a
compound composed of copper and chlorine. A weighed quantity of
copper chloride is dissolved in distilled water. The resulting copper
chloride solution is then allowed to react with solid zinc, Zn, to form
insoluble copper, which is filtered, dried, and weighed. From the
experimental data you can calculate the percentage composition and the
ratio of moles of copper to moles of chlorine in the compound and thus
determine the empirical formula.
Pre – Lab Questions
A student wanted to determine the empirical formula of an iron chloride
compound. The student performed an experiment similar to the one
described in the procedure of this laboratory exercise and collected the
following data.
Name: ___________________________________________________ Date: _________
Table 1.
Mass
Mass
Mass
Mass
Mass
of
of
of
of
of
Measurement
iron chloride (g)
magnesium before reaction (g)
magnesium after reaction (g)
filter paper (g)
filter paper and iron (g)
Mass
5.000 g
3.159 g
2.201 g
1.134 g
3.337 g
Perform the calculations listed below. Show each calculation clearly in
your notebook to receive credit for these calculations.
1. Calculate the mass of magnesium used in the reaction.
2. Calculate the number of moles of magnesium used in the reaction.
3. Calculate the mass of iron formed during the reaction.
4. Calculate the number of moles of iron formed during the reaction.
5. Write an equation for the reaction that occurred between the
magnesium metal and the FexCly sample.
6. Assuming that magnesium is the limiting reactant in this lab,
calculate the mass of chlorine in the original iron chloride sample.
7. Calculate the number of moles of chlorine in the original iron chloride
sample.
8. Calculate the ratio of the moles of chlorine/moles of iron.
9. Calculate the empirical formula of the original iron chloride
10. Calculate the percent composition of iron and of chlorine in your
final product.
Procedure
This experiment will be run in duplicate. The two trials should be
performed simultaneously in order to save time. Record all experimental
data in your notebook in a data table.
1. Label two clean 50 mL beakers #1 and #2.
2. Place beaker #1 on the balance and weigh a 1.5 g sample of copper
chloride compound into the beaker.
3. Record the mass of the sample in your notebook.
4. Repeat the process for beaker #2. If any copper chloride should spill in
the process of weighing, clean it up and place in the waste container
in the lab.
5. Measure out 25 mL of distilled water using a graduated cylinder.
6. Place the distilled water into beaker #1 and stir to completely dissolve
the solid copper chloride sample.
7. Repeat step 6 for beaker #2.
8. Obtain a piece of Zn and obtain its mass. Record this mass in your
data table.
9. Handling the Zn with tongs, add it to beaker #1.
10. Repeat steps 8 and 9 for beaker #2.
11. Use the stirring rod to periodically scrape solid Cu from the Zn piece
into the solution.
12. After the reaction has stopped, add 5-10 drops of HCl solution and
stir.
13. Using tongs, remove the Zn from the beaker, making sure to leave
behind adhering Cu.
14. Dry the Zn with a paper towel and obtain its mass, then dispose of
the Zn in a container under the fume hood.
15. Carefully decant (pour off) the supernatant liquid from the solid Cu
into a waste beaker.
16. Wash the solid Cu by adding about 10 mL distilled water to it,
stirring vigorously, then decanting the rinse water into your waste
beaker. This will remove dissolved ZnCl2.
17. Add about 10 mL isopropyl alcohol (isopropanol) to the Cu, stir
thoroughly, and decant the alcohol into the waste beaker. Repeat
this step twice more.
18. Obtain two pieces of filter paper and label them #1 and #2 using a
pencil.
19. Record the mass of each piece of filter paper in your data table.
20. Fold the filter paper and place it in a funnel.
21. Transfer the Cu from the reaction beaker to the filter paper. Make
sure to transfer every trace of solid Cu. You may use isopropanol
to rinse the Cu from the beaker if necessary.
22. Place the filter paper on a piece of paper towel.
23. Mark the paper towel with your initials and place it in the
appropriate container.
24. When the filter paper is completely dry, weigh the filter paper and
solid for each trial. Record the masses in your data table.
Results
Record all data and observations in the appropriate data table in your
laboratory notebook.
Data Analysis
Perform the calculations listed below. Record your results in Table 2.
Show each calculation clearly in your notebook to receive credit for these
calculations.
1. Calculate the mass of zinc used in the reaction.
2. Calculate the number of moles of zinc used in the reaction.
3. Calculate the mass of copper formed during the reaction.
4. Calculate the number of moles of copper formed during the reaction.
5. Calculate the mass of chlorine in the original copper chloride sample.
6. Calculate the number of moles of chlorine in the original copper
chloride sample.
7. Calculate the ratio of the moles of chlorine/moles of copper for each
trial.
8. Calculate the average ratio of the moles of chlorine/moles of copper.
9. Calculate the empirical formula of the original copper chloride
10. Calculate the percent composition of copper and of chlorine in your
final product.
Conclusion
1. If your sample of copper chloride had not been dried thoroughly,
would this cause the ratio of moles of copper to moles of chloride to
be too high or too low? How would this affect the empirical formula?
Indicate your reasoning clearly.
2. How can the percent composition be used to determine the mole
ratio?
3. Write a balanced chemical equation to show what happened during
this laboratory exercise.
4. What color was the copper chloride solution at the start of the
reaction?
5. What color was the zinc chloride solution at the end of the reaction?
Table 1. Mass data for reaction of copper chloride with zinc
Copper chloride sample # ________________
Trial 1
Trial 2
Mass of copper chloride (g)
Mass of zinc before reaction (g)
Mass of zinc after the reaction (g)
Mass of filter paper
Mass of filter paper and copper (g)
Table 2. Calculations for the empirical formula of copper chloride
Trial 1
Mass of zinc used in reaction (g)
Moles of zinc used in reaction (g)
Mass of copper formed (g)
Moles of copper formed (g)
Mass of copper chloride reacted (g)
Mass of chlorine in copper chloride
sample (g)
Moles of chlorine in copper chloride
sample (g)
Ratio: moles of chlorine/ moles of
copper
Average ratio: moles chlorine / moles
copper
Empirical formula of copper chloride
(Careful rounding off!!)
Trial 2