Percent Composition and Empirical Formula
Based on an experiment written by
Dr. Anne Glenn
Mr. David MacInnis, Jr.
Dr. Rob Whitnell
Guilford College
Purpose
The purpose of this experiment is to determine the percent copper in a compound containing only copper
and chlorine by reducing the copper(II) ions to metallic copper with zinc by the following oxidation (redox)
reaction:
2+
2+
Cu (aq) + Mg(s) → Cu(s) + Mg (aq)
The solid metallic copper will be collected by filtration, washed, dried and weighed. You will then be able
to calculate the mass percentages (percent composition) of copper and chlorine in the compound. From
this you will be able to determine the empirical formula of your original copper containing compound. This
experiment will demonstrate the technique of reducing a compound to its elements in order to determine
its empirical formula.
The Details
Chloride-containing salts of copper are soluble in water.
2+
1-
CuClx(s) + H2O(l) → Cu (aq) + x Cl (aq)
2+(
The aqueous copper(II) ion (Cu aq)) exhibits a characteristic blue color that can provide visual cues as
to the extent of any reaction involving that ion.
The copper(II) can then be reduced to elemental copper in an oxidation-reduction (or cation
displacement) reaction as shown above. Any excess solid elemental magnesium can be removed from
2+
the flask by oxidizing it to Mg (aq) using an acid such as hydrochloric acid:
1+
2+
Mg(s) + 2 H (aq) → H2(g) + Mg (aq)
Assuming the initial mass of the copper-chlorine sample known, and assuming all of the copper from that
sample is collected in the form of elemental copper, it is possible to calculate the mass of chlorine in the
original sample, and, in turn, the percentage composition of those two elements in the original sample. It
is also possible to convert the masses of copper and chlorine to the number of moles of those two
elements in the original sample, and, in turn, the mole ratio of chlorine to copper in the compound. That
mole ratio can be easily converted to the empirical formula for the compound.
Safety Information
6M HCl is a strong acid. Gloves are recommended when handling it. If you spill some on you wash it off
immediately with a large quantity of water. Large spills in your laboratory area should be treated with
sodium bicarbonate. Be sure to ask your lab instructor for assistance.
Ethanol and acetone are extremely flammable. Do not use any open flames during this experiment.
Equipment
Two 250 mL beakers
100 mL graduated cylinder
Two stirring rods with rubber policemen (scrapers)
Filter paper
Two funnels
Two 250 mL flasks for collecting the copper filtrate
Vial of copper-chlorine compound
Magnesium turnings (≈ 1.5 g)
Dropper bottle of 6M HCl (10 to 15 mL total)
Dropper bottles of ethanol and acetone
Procedure
1. Obtain an approximately 4 g sample of the copper chlorine compound. Divide the sample between
two clean 250 mL beakers (tared and labeled), recording the mass of the compound each time.
2. Place a clean stirring rod with rubber policemen in each beaker and leave them there. Add
approximately 50 mL of distilled water to each beaker and then add 2 mL (approximately 40 drops) of
the 6M HCl. Stir until the compound has completely dissolved. If solid residue remains after 5 minutes
of stirring add 6M HCl in 2 mL increments stirring 1 min. each until the solid has dissolved. Be sure
to record the color of these solutions.
3. Add approximately 0.7 g of magnesium turnings and stir well. Let the mixture stand with frequent
stirring for approximately 15 min. after which there should be a deposit of metallic copper on the
bottom of the beaker. The solution should be colorless and the evolution of hydrogen gas should be
slow. If the solution becomes cloudy during this time add 6M HCl drop by drop with stirring until the
solution clears.
4. Label with pencil and weigh two individual pieces of filter paper.
5. To destroy any excess metallic magnesium that might be present in the solution add 2 mL of 6M HCl
to each beaker and stir. If hydrogen gas is not evolved you probably did not have enough magnesium
present to reduce all the copper. In this case you should add another 0.2 g of magnesium to the
solution and stir for 5 minutes. In either case when the gas evolution becomes slow add about 0.5 mL
of 6M HCl to each beaker and stir until no more gas is evolved.
6. Filter each solution through its own weighed filter paper. Use your rubber scraper to ensure that you
transfer all the copper solid to the filter (be sure to rinse all the copper from the beaker into the filter
paper with distilled water as required). Wash each filter with approximately 100 mL of distilled water
to clean the copper precipitates.
7. Wash the filters containing the solid copper with 10 mL of alcohol to remove all the water. Wash again
with 10 mL of acetone to completely remove the water and spread the filter papers on a paper towel
and allow them to dry. The solution in your flask is known as the filtrate. For this lab it can be washed
down the sink with plenty of water.
8. When the filter papers containing the recovered elemental copper have completely dried, weigh each
one. User your data to calculate the mass percentages of copper and chlorine in the original
sample. Use this information to determine the empirical formula of the compound. After you are
confident that you have collected and weighed the copper correctly, the solid copper and filter paper
can be disposed of in the trash.
Data
Trial #1
Trial #2
Trial #3
Mass of empty beaker
_______
_______
_______
Mass of beaker plus CuClx
_______
_______
_______
Mass of clean, dry filter paper
_______
_______
_______
_______
_______
_______
o
Mass of filter paper plus dry Cu
Calculations
Please complete the calculations using Excel. Be sure to do all calculations in terms of cell
addresses. Please name the file CuClx_AB where AB represent your first and last initials. Be
prepared to submit the spreadsheet as required by your laboratory instructor.
1. Calculate the mass of elemental copper recovered in each successful reaction.
2. Calculate the mass of chlorine that had been combined with the copper in each successful reaction.
3. Calculate the percent by weight of copper and chlorine present in each trial. If these percentages are
significantly different it may be necessary to do a third trial.
4. Calculate the number of moles of copper used in each successful reaction. Round the answer off to
3 significant figures. Use 63.55 gm as the mass of 1.00 mole of copper.
4. Calculate the number of moles of atomic chlorine that reacts with the copper in each successful
reaction. Round the answer off to 3 significant figures. Use 35.45 gm as the mass of 1.00 mole of
atomic chlorine.
5. From the results of calculations 3 and 4 determine the empirical formula for the compound. Use 3
significant figures in these calculations and round off answers to 3 significant figures. Divide the
results of the calculations in 3 and 4 by the smaller of the two numbers. Convert these results into
integers by either rounding or by multiplying both by the same integer.
6. The empirical formula for this copper chloride (CuClx) is _________________.
Additional questions
1. In step three of the procedure, you added 0.7 g of magnesium metal to the solution containing the
copper-chlorine compound. How many moles of magnesium is this and how does it compare to the
number of moles of your copper chlorine compound? Why do you have to use so much
magnesium?
2. What error would result if you did not add excess hydrochloric acid to dissolve the magnesium in the
sample?
3. It is important to analyze what errors or other uncontrollable factors might have affected your results
and how your results might have changed had these errors been corrected. For each of the following,
state how the error affects the mass percentage of the copper in your compound and in turn the
empirical formula.
a. The original sample of the compound was wet.
b. An impurity containing neither copper or chlorine was present in your sample of the compound.
c. Not all of the excess magnesium was oxidized.
d. The copper was not completely dry when you weighed it.