Lab 5.
Chemical Reactions of Copper
Purpose
The purpose of this lab is to transform copper metal into a number of different substances and
then back into copper metal through a series of chemical reactions.
Learner Outcomes
Ø You will learn how to perform more experiments that involve mixing, heating, vacuum
filtration and the use of centrifuges.
Ø You will learn more about how chemical reactions work and how to determine the
theoretical, actual and percent yields of a chemical reaction.
Ø You will begin to develop the important skill of interconverting amounts of substances
between the three common ways we measure them in chemistry: grams, moles, and number
of formula units.
Safety information
In the course of this lab, you will be using some dangerous chemicals and some procedures must
be done in the fume hood. The concentrated nitric acid (HNO3) is extremely corrosive and must
be used with care and in the hood at all times. The concentrated HCl, 6 M HCl, 1 M HCl, 3 M
H2SO4, and 6 M NaOH are all toxic and corrosive and must be handled carefully. Use the
labeled 10 mL graduated cylinder next to the reagent bottles for the acids. Do not use your own
graduated cylinders for these compounds. All the acids (chemicals that start with the symbol 'H')
also produce fumes that you must avoid by keeping the open containers away from your face at
all times. During the first reaction the brown gas, NO2, is produced. This gas is toxic and the
reaction must be done in the fume hood. Even though these are all serious chemicals, they can
all be used safely if you follow the correct procedures. You must wear goggles at all time. If
you have to be told more than once during the lab to put your goggles on, you will be docked
half a point for each reminder. If you continue to remove them you will not be able to do the lab
and receive no points for it. When you are seated at a desk in the front of the room, you can
remove your goggles. Closed-toed shoes, no food, drinks, gum, earphones, or hoods are
allowed. You can keep any food or drink in your bag or pack. If there’s a break in your
experiments, you can wash your hands and eat or drink something outside. Bring your copy of
the safety rules with you to lab every week.
The table below gives basic information about the chemicals used during the lab that one must be
aware of:
Chemical
Chemical symbol
Toxicity information
concentrated HNO3
HNO3
toxic, oxidant, corrosive
must be used in fume hood
nitrogen dioxide (produced in first
NO2(g)
toxic gas
reaction)
must be produced in fume hood
6 M and 1 M sodium hydroxide
NaOH(aq)
toxic and corrosive
3 M sulfuric acid
H2SO4(aq)
toxic, oxidant, corrosive
concentrated HCl; 1 M and 6 M HCl HCl(aq)
toxic and corrosive
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Materials
Chemicals
Cu mesh
concentrated HNO3
6 M NaOH
3 M H2SO4
Mg ribbon
concentrated HCl
ethanol in dropper bottle
Equipment
small weigh boats
2 100 mL beakers
1 250 mL beader
10 mL graduated cylinder
glass rod
red litmus paper
ring stand
2 rings
Bunsen burner with tubing
wire gauze
beaker tongs
2 centrifuge tubes (12 mL each)
centrifuge
Buchner funnel
7 cm filter paper
filter flask
utility clamp
wash bottle
vacuum tubing
evaporating dish
Waste and Cleanup
All liquid waste is to be poured into the 4L container labeled “Discarded supernatant liquids and
filtrates”. At the end of the experiment, your solid Cu waste goes into the plastic tub labeled
“Dry discarded copper”. Follow the 3 step rinsing procedure described below for your glassware
and equipment that has come into contact with chemicals.
At the end of your experiments, clean all glassware, dry it the best that you can, and put it back
in your drawer. Whenever you rinse glassware to clean it, rinse once with tap water and pour it
out in the proper waste container. Rinse again with tap water and pour down the sink. And then
rinse a third time with de-ionized (DI) water. All the faucets on the right side of the sinks are DI
water faucets. If the contents of the glassware are not water soluble, you may have to use soap
and a test tube brush to clean it. Then rinse thoroughly. Clean the area around your lab bench
and make sure that the balance area, the area where the chemicals are located, and the fume
hoods are clean as well. Lock your drawer and pull on it to make sure it’s locked (you are
financially responsible for its contents) and then wash your hands with soap and water. After
this, you can take a short break and then sit down to work on the data analysis and postlab
questions.
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Background information
In its pure elemental form, copper is a metal and is written as Cu or Cu(s). It has atomic number
29 meaning that its nucleus contains 29 protons. In it elemental form, it also has 29 electrons so
that the atom has no net electrical charge. However, since it is a metal, it can lose electrons to
become a cation and combine with nonmetal or polyatomic anions to form a variety of ionic
compounds. Copper is a transition metal and can lose either 1 electron to become a Cu+ ion or 2
electrons to become a Cu2+ ion. Ionic compounds can be made from either of these ions of Cu.
For instance, copper(I) oxide has the formula Cu2O and copper(II) oxide has the formula CuO.
These are completely different chemicals; Cu2O is a red solid and CuO is a black solid. Last
week when you heated pure Cu metal is the air, the oxide you produced was CuO.
The reactions you will be performing this week are more complex to write chemical equations
for and are not as easily categorized into the 5 reaction types we studied last week.
Reaction 1:
Cu(s) + 4HNO3(aq) à Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)
Here, copper metal reacts with concentrated nitric acid to produce aqueous copper(II) nitrate,
nitrogen dioxide gas, and water. This is a dramatic reaction with the copper metal dissolving to
form the greenish copper(II) nitrate solution and the evolution of the brown nitrogen dioxide gas.
Since NO2(g) is toxic, this reaction is always performed in a fume hood.
Reaction 2:
Cu(NO3)2(aq) + 6H2O(l) à Cu(H2O)6(NO3)2(aq)
When water is added to the product mixture from Reaction 1, the cation [Cu(H2O)6]2+ is formed.
This is known as hexaaquocopper(II) ion. It is not uncommon for transition metal cations bond
with water molecules forming these more complex cations. This ion is a bright blue color
forming a bright blue, clear solution.
Reaction 3:
Cu(H2O)6(NO3)2(aq) + 2NaOH(aq) à Cu(OH)2(s) + 2NaNO3(aq) + 6H2O(l)
The addition of sodium hydroxide results in an exchange reaction. Notice that the product
copper(II) hydroxide is written as a solid (s). This indicates that it is insoluble in water and the
resulting solid that appears is known as a precipitate. Exchange reactions often result in
products that are insoluble in water and thus form precipitates. True aqueous solutions may or
may not be colored but they are always clear. If an aqueous solution becomes cloudy or opaque
(again of any color) this indicates the presence of a solid precipitate. In this case, Cu(OH)2(s) is
present as a light blue precipitate.
Reaction 4:
Cu(OH)2(s) ⎯heat
⎯⎯→ CuO(s) + H2O(l)
The addition of heat to the light blue Cu(OH)2(s) precipitate results in a decomposition reaction
in which copper(II) oxide and water are formed. Note that this is the same compound that you
produced last week by heating copper metal in air is produced by a different chemical reaction.
Copper(II) oxide is also insoluble in water but it is black in color so one observes the
transformation of the light blue Cu(OH)2 precipitate to a dark brown or black precipitate in the
course of the reaction. Another feature of this chemical equation is the use of the word 'heat'
over the arrow. The application of heat can also be indicated by a triangle rather than the word
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Δ CuO(s) + H O(l). Other reaction conditions are sometimes indicated
'heat': Cu(OH)2(s) ⎯⎯
2
→
in a similar way. For instance, water can be split into H2(g) and O2(g) by the application of
electrolysis, the application of an electrical current: H2O(l) ⎯electrolysis
⎯ ⎯ ⎯⎯→ H2(g) + 1/2O2(g).
Reaction 5:
CuO(s) + H2SO4(aq) + 5H2O(l) à Cu(H2O)6SO4(aq)
When sulfuric acid is added to the copper(II) oxide precipitate formed in the previous reaction,
the hexaaquocopper(II) ion, [Cu(H2O)6]2+, with its characteristic clear blue color is again formed,
this time with the sulfate ion as the counter ion. In the course of this combination reaction, the
copper(II) oxide precipitate dissolves yielding a clear blue solution.
Reaction 6:
Cu(H2O)6SO4(aq) + Mg(s) à Cu(s) + MgSO4(aq) + 6H2O(l)
Here, magnesium displaces the copper in a displacement reaction resulting in a precipitate of
pure copper metal and aqueous magnesium sulfate and water. At this point, we have come full
circle with copper having been transformed several times into different substances and finally
regenerating pure copper metal. At this point, we need to do a few more things to separate the
Cu(s) from the aqueous magnesium sulfate and any other excess reactants and products that may
be present from the earlier reactions. These steps will be described in the Procedure section.
Theoretical yield, actual yield, and percent yield
One might expect that the quantity of copper at the end of Reaction 6 to be the same as the
amount of copper weighed out at the beginning. The law of conservation of mass might lead us
to this conclusion since it tells us that copper atoms cannot be created or destroyed in any
chemical reaction. In practice, however, one loses a little bit of copper during most steps of the
reaction sequence. In the process of transferring solutions or precipitates from one receptacle to
another, small amounts of product are always lost. The more careful one is and the better one's
lab techniques, the less copper one will lose. Thus, a comparison of the amount of copper metal
at the beginning and the end of the experiment is a measure of one's laboratory skills. Let's say
that a student starts out with 0.245 g Cu(s). In an absolutely perfect world, she would end up
with 0.245 g Cu(s). This theoretical yield is said to be 0.245 g. It is what is theoretically
possible. Let's say this is a real student, however, and that she ends up with 0.233 g of Cu(s) at
the end. This is the actual yield. It is the yield that she actually got in the experiment. The final
measure of her lab skills is the percent yield:
actual yield
percent yield =
x100% .
theoretical yield
0.233g
x100% = 95.1% .
This student had a % yield of:
0.245g
Interconverting between grams, moles, and number of formula units.
In the coming weeks we learn about stoichiometry. This is the concept that chemicals combine
in fixed numerical relationships. For example, Reaction 1 is:
Cu(s) + 4HNO3(aq) à Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)
This states that 1 atom of copper reacts with 4 formula units of nitric acid to produce 1 formula
unit of copper(II) nitrate, 2 molecules of nitrogen dioxide, and 2 molecules of water. Note that
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we use the term ‘formula unit’ for a single unit of an ionic compound and ‘molecule’ for a single
unit of a molecular compound. There is no such thing as a ‘molecule’ of Cu(NO3)2 since it really
consists of a crystalline array of copper(II) and nitrate ions in a 1:2 ratio, not a distinct molecule.
We can also interpret this equation to read: 1 mole of copper reacts with 4 moles of nitric acid
and produces 1 mole of copper(II) nitrate, 2 moles of nitrogen dioxide, and 2 moles of water.
Looking at the periodic table, we see that the atomic mass of Cu is 63.55. This means that we
can go into a lab and weight out 63.55 g of Cu on a balance and we would know that we have 1
mole of Cu, or 6.022 x 1023 atoms of Cu. This concept of the mole (also known as Avogadro’s
number) allows us to know the number of particles (atoms, molecules, formula units, or ions) of
a sample of a substance by weighing it out on a laboratory balance. The formula weight of
HNO3 is obtained by adding up the atomic weights of the constituent atoms. It’s formula weight
is 63.02 amu. Thus, if we weigh out 63.02 g of pure HNO3 on a lab balance, we know that we
have 1 mole of HNO3. From the balanced chemical equation above, we see that we need 4 moles
of HNO3 to react with 1 mole of Cu. 4 x 63.02g = 253.1 g so this is how much HNO3 we would
need to react with 1 mole of Cu. In order to determine the amount of each reactant required in
each step of this lab, it was necessary to determine the number of moles of each substance
required based on the balanced chemical equations. Using the formula weights, we then
converted these moles to grams. The amounts used in the procedure were determined in this
way. This concept of stoichiometry is one of the corner stones of this course so it is necessary to
become proficient in interconverting between grams, moles, and number of units. This will soon
come in handy in order to calculate how many grams of reactant we need for a particular
reactions, and how many grams of product are produced. By being able to predict how much
product is going to be produced in a given chemical reaction, we are using the full power of
balanced chemical equations.
Procedure
Reaction 1:
Cu(s) + 4HNO3(aq) à Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)
• To begin the reaction sequence, weigh out about 0.2 g of Cu(s) in a weight boat. To do this,
make sure the balance reads 0.000 g and place a plastic weigh boat carefully on the balance
pan. Hit the 'Tare' button on the balance to zero it. Then carefully add about 0.2 g of Cu(s)
to the weigh boat. Record the precise mass of the copper (that is, record all the digits
displayed on the balance) on your data sheet. Make sure that the mass is in the range of
0.190 to 0.210 g.
• Transfer the Cu(s) to a clean, dry 100 mL beaker and take it to a fume hood containing a
bottle of concentrated nitric acid.
• Being very careful, pour 2 mL of concentrated HNO3 into the 10 mL graduated cylinder
provided in the hood. Make sure that you have more than 1.8 mL but less than 2.2 mL of the
acid.
• Also very carefully, pour the concentrated nitric acid into the beaker containing the Cu(s),
keeping the beaker well within the hood. At this point, brown NO2(g) will be produced. The
reaction is complete when no more brown gas is observed and all the Cu has been dissolved.
If any copper remains, see your instructor.
• At this point, your solution should be an emerald green color from the Cu2+ ions.
• Keep the beaker in the hood.
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Reaction 2:
Cu(NO3)2(aq) + 6H2O(l) à Cu(H2O)6(NO3)2(aq)
• Fill your own 10 mL graduated cylinder with 5 mL (±0.2 mL) of DI water. Do not use tap
water.
• Take the DI water to the hood and add it to slowly to the beaker. The blue color of the
hexaaquocopper(II) ion, [Cu(H2O)6]2+, should immediately appear. Swirl the contents
gently.
• You can now take the beaker back to your lab bench.
Reaction 3:
Cu(H2O)6(NO3)2(aq) + 2NaOH(aq) à Cu(OH)2(s) + 2NaNO3(aq) + 6H2O(l)
• Using your 10 mL graduated cylinder, measure out roughly 5 mL (±1 mL) 6 M NaOH
• Transfer this NaOH solution to a 2nd clean 100 mL beaker.
• Using a dropper pipet, start adding the NaOH solution drop by drop to the beaker containing
the [Cu(H2O)6]2+ solution. Make sure to stir the mixture thoroughly and continuously with a
glass stirring rod as you are adding the NaOH solution. Soon you will observe the light blue
precipitate, copper(II) hydroxide.
• After you've added about 1 mL of NaOH, test the reaction by placing a drop of the mixture
on a piece of red litmus paper using your glass stirring rod. Litmus is red when tested with
an acidic solution and blue when tested with a basic solution. Because you added an excess
of concentrated nitric acid for Reaction 2, your solution is highly acidic and the litmus paper
should remain red. But since you are now adding NaOH (a base) as a reactant for this step,
once you've added enough NaOH to complete the reaction, the NaOH will start to neutralize
the excess acid. When all the acid has been neutralized, the solution starts to become basic
as more NaOH is added and thus turn the litmus paper blue. Continue adding the NaOH
solution drop by drop while stirring thoroughly and testing the basicity of the mixture
periodically on the red litmus paper. It is critical that you stir thoroughly before the litmus
test since if your glass stirring rod picks up a drop of unmixed NaOH solution, your test
could falsely indicate a completed reaction. As soon as you get a positive litmus test (the red
litmus paper turns blue), however, stop adding NaOH solution. Make sure you do not add too
much NaOH solution as this will mess up the subsequent reactions. Your reaction is now
complete and should have a significant amount of light blue precipitate, Cu(OH)2(s).
Reaction 4:
Cu(OH)2(s) ⎯heat
⎯⎯→ CuO(s) + H2O(l)
• Fill your cleaned 10 mL graduated cylinder with DI water to the 10 mL (±0.5 mL).
• Add the water to your reaction beaker.
• Assemble a ringstand with ring and wire gauze to heat the contents of the beaker. Your
instructor will show you the correct setup.
• Heat the mixture until boiling and boil for 1-2 minutes while stirring with a glass rod. You
should notice the light blue Cu(OH)2 precipitate turn into a dark brown or black CuO
precipitate. If this transformation is not finished after 2 minutes of boiling, continue heating
until no evidence of copper(II) hydroxide remains.
• Turn off the Bunsen burner and let the beaker cool to room temperature.
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•
•
•
•
•
•
Transfer the contents of the beaker into 2 clean centrifuge tubes. Make sure to rinse any
precipitate left in the beaker into the centrifuge tubes using as little DI water as possible.
This ensures that you have transferred as much copper compound into the tubes as possible
without exceeding their capacity. The volume of the liquid and the precipitate should be
roughly evenly split between the two tubes.
Centrifuge the tubes for 1 minute as described by the instructor. Make sure that the
centrifuge is balanced with opposing centrifuge tubes before turning it on.
Upon removing the tubes from the centrifuge, you will see that the CuO(s) has been pushed
to the bottom of the centrifuge tubes by centrifugal force. The liquid above the precipitate is
called the supernatant liquid. Pour off the liquid into the waste container labeled “Discarded
supernatant liquids and filtrates”. This process in known as decanting the liquid.
Put approximately 30 mL of DI water into a clean 150 mL beaker and heat it to boiling.
Using beaker tongs (not tongs or paper towels) to lift the beaker of boiling water, fill the
centrifuge tubes to within 1 cm of the opening with the boiling water and stir with the glass
stirring rod. This cleans the CuO precipitate from dissolved substances present from the
series of reactions.
Centrifuge these tubes for 1 minute again and decant the supernatant liquid into the waster
container as before. You now have fairly pure (but wet) CuO(s) in the centrifuge tubes.
Reaction 5:
CuO(s) + H2SO4(aq) + 5H2O(l) à Cu(H2O)6SO4(aq)
• Put about 2.5 mL (±0.2 mL) of 3 M H2SO4 into the 10 mL graduated cylinder provided.
• Carefully transfer this into one of the centrifuge tubes and stir with a glass rod. The
copper(II) oxide will start to dissolve forming the clear blue solution characteristic of the
[Cu(H2O)6]2+(aq) ion.
• Similarly, add 2.5 mL of 3 M H2SO4 into the other centrifuge tube and stir.
• If one of the tubes still has some CuO precipitate in it, pour the contents of the other
centrifuge tube into the one containing the remaining CuO precipitate and stir until all
precipitate is gone.
• Pour the now clear solution of Cu(H2O)6SO4(aq) (and some excess H2SO4(aq)) into a clean
150 mL beaker and add 15 mL of DI water.
Reaction 6:
Cu(H2O)6SO4(aq) + Mg(s) à Cu(s) + MgSO4(aq) + 6H2O(l)
• Weigh out 0.4 (±0.05g) Mg using the lab balance, a clean weigh boat, and the tare feature of
the balance.
• Add this to the beaker containing the hexaaquocopper(II) ion.
• This single displacement reaction results in the precipitation of metallic Cu(s), our final
product.
• This reaction may take several minutes. Stir the mixture vigorously from time to time and
break up the solid with a glass stirring rod to ensure that the surface of the magnesium metal
has access to the solution. During this reaction, you will see bubbles of hydrogen gas. This
forms due to the excess of H2SO4 from Reaction 5. The chemical equation for this side
reaction is:
Mg(s) + H2SO4(aq) à MgSO4(aq) + H2(g).
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• If any blue color remains after several minutes and no more bubbling occurs, you have run out
of Mg(s). Add a little more Mg to complete the reaction.
• Once the reaction is complete, there will be excess Mg metal. Add a few drops of
concentrated HCl from the dropper bottle in the hood. You should detect the reaction:
Mg(s) + 2HCl(aq) à MgCl2(aq) + H2(g) by the presence of bubbles of hydrogen gas. Once
the bubbling has stopped add a few more drops of HCl(aq). This may take several minutes.
If no bubbling occurs, you have gotten rid of any excess Mg metal. Make sure to stir up the
mixture with a glass stirring rod from time to time.
At this point the only solid in your beaker should be Cu metal. However, there will be some
MgSO4(aq), MgCl2(aq), HCl(aq), and several other dissolved substances from the previous
reactions. To finish the experiment and determine how much copper has been recovered, we
need to clean the Cu(s) from these dissolved substances and dry the metal. We will accomplish
both these things using the technique of vacuum filtration.
• Assemble a vacuum filtering apparatus using a Buchner funnel, filter paper, a filter flask,
vacuum tubing, a neoprene adapter, a ring stand, and a utility clamp. You instructor will
demonstrate this assembly.
• With the filter paper firmly in place on the bottom of the Buchner funnel, moisten it
completely with a few drops of DI water from a wash bottle.
• Turn the vacuum on full and transfer all the contents of the beaker containing your product
into the Buchner funnel. Make sure to rinse any traces of Cu(s) remaining in your beaker
with a small quantity of DI water from your wash bottle to minimize loss.
• Measure out roughly 5 mL of DI water using your clean 10 mL graduated cylinder and
carefully rinse the Cu on the filter paper in the funnel with the vacuum pulling. Try to wash
all the Cu.
• Do this rinsing 2 more times using 5 mL of DI water each time. At this point you have
separated the Cu from any dissolved substances and the only thing left to do is dry it. The
liquid in the filter flask is known as the filtrate and will be discarded.
• Measure out 5 mL of ethanol into a clean, relatively dry 10 mL graduated cylinder and pour
it carefully over the Cu making sure to reach all the Cu. Ethanol will mix with the water
left adhered to the Cu and help dry it faster.
• Keep the vacuum on until all the Cu and the filter paper is dry. This may take some time
but it is essential to remove all water before weighing.
• While the Cu is drying, put a clean, dry evaporating dish on the wire gauze heating
apparatus and heat it using the Bunsen burner for two minutes. Let it cool completely to
room temperature. You can now be sure that the evaporating dish is completely dry. Don’t
touch it; use a paper towel to carry the evaporating dish.
• Weigh the evaporating dish and record its mass on the data sheet.
• Carefully remove the filter paper from the Buchner funnel and transfer as much of the dry
Cu as possible to the evaporating dish. Be careful not to scrap the filter paper to the point
that bits of filter paper are transferred with the Cu.
• Weigh the evaporating dish containing the dry Cu and record its mass.
• Your instructor may ask you to heat the evaporating dish on a hot plate to ensure it is dry.
• Discard the filtrate into the waster container labeled “Discarded supernatant liquids and
filtrates”.
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• Rinse your vacuum flask using the 3 step rinsing process and dry it as best as possible.
• When you are completely sure that your Cu is dry and that your results are final, discard the
Cu into the waste container in the hood labeled “Dry discarded copper”.
• Make sure all your glassware and other equipment is clean and as dry as possible before
putting it away and locking your drawer.
Calculations
• Determine the mass of recovered copper by subtracting the mass of the clean dry evaporating
dish from the mass of evaporating dish + copper determined at the end of Reaction 6.
• Calculate the percent yield of Cu for this experiment.
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Prelab Assignment for Lab 5: Chemical Reactions of Copper
Name: ____________________________ Date: __________ Lab section: _____
Show your work when there are calculations, write units, and use the correct significant figures.
1) What is the purpose of this lab? Use full sentences and your own words to answer this
question.
2) What is the name of the substance with formula Cu(H2O)6SO4?
3) Student X weighs out 0.195 g of Cu metal.
a) How many moles is this?
b) How many copper atoms is this?
4) Student X does the entire experiment, making sure that his copper product is bone dry at the
end. He weighs his product and he finds that he has 0.189 g of Cu.
a) What is his theoretical yield in grams?
b) What is his actual yield in grams?
c) What is his percent yield? Use the correct number of significant figures.
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Data Sheet for Lab 5: Chemical Reactions of Copper
Name: ____________________________ Date: __________ Lab section: _____
Starting mass of Cu in grams:
________________
Mass of dried evaporating dish in grams:
________________
Mass of evaporating dish + Cu in grams:
________________
Mass of recovered Cu in grams:
________________
Percent yield of Cu:
________________
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