The Determination of a Chemical Formula
In this experiment, you will use the law of definite proportions to find the chemical formula for a hydrated
compound containing copper, chlorine, and water molecules locked in the crystal structure of the solid
compound. The general formula for the compound is CuxCly•zH2O, and its name is copper chloride hydrate
(hydrate means that there is water trapped inside the actual crystals). The letters x, y, and z represent integers
that will establish the proper chemical formula for this substance. First, you will gently heat a sample of the
compound to drive off the water of hydration. By measuring the mass of the sample before and after heating
you can determine the amount of water in the sample and identify the value of z.
Second, you will conduct a chemical reaction with the dried sample, which will produce elemental copper.
By measuring the mass of copper that forms, you will have the necessary information to determine the moles
of copper and chlorine in your sample, and you will be able to establish the proper chemical formula.
PRELAB
To complete the prelab, go to the following websites and then use the information on those sites to answer the
following questions. They must be completed prior to entering lab.
http://www.hydrateweb.org/what-hydrate
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Hydrate.html
1. An analogy of a water balloon is often used to describe a hydrate. Pretend you are explaining what a hydrate is to
another student and use the analogy of the water balloon to help them understand.
2. An example of a hydrated crystal is CoCl2·6H2O. This crystal consists of a cobalt ionically bonded to two chlorides.
Then there are 6 molecules of water trapped within the crystal. Determine the molar mass of the hydrated cobalt(II)
chloride hexahydrate (CoCl2·6H2O).
http://preparatorychemistry.com/Zn_CuSO4_flash.htm
3. The above site shows the reaction that will be happening in our lab. We will place a piece of zinc metal in a
solution of copper(II) sulfate. What ion specifically will be reacting with the the Zn atoms? And what will it be
turning into?
4. Draw a before and after picture of what the solution looks likes before the reaction starts and what it will look like
after a few interactions of the copper ion with the zinc atoms.
MATERIALS
Crucible with cover
Crucible tongs
Scoopula
Filter paper
Hot plate
150 mL beaker
Stirring rod
Funnel
Unknown solid copper chloride
hydrate
Aluminum wire, 20 gauge
6M hydrochloric acid solution
95% ethanol solution
Distilled water
Wash bottle
Balance
Watch glass
10 mL graduated cylinder
PROCEDURE
Day 1:
1. Obtain and wear goggles.
2. Measure and record the mass of a clean, dry crucible without cover. Obtain about ___ g (btw 0.5-2.0g) of
the unknown copper chloride hydrate and place it in the crucible. Use a scoopula to break up any large
pieces of the substance by pressing the pieces against the wall of the crucible. Measure and record the
mass of the crucible with compound.
3. Set up a hot plate and turn the heat setting to approximately 8.
4. Place the crucible on the hot plate and begin heating. After a few minutes of heating, note the color
change, from blue-green to brownish, as the water of hydration is driven out of the crystals. When the
sample has turned brown, gently heat (on a setting of 5) the crucible for two more minutes.
5. Remove the crucible carefully and turn off the hot plate. Cover the crucible and allow the sample to cool
for about five minutes…you should be able to pick up the crucible without burning yourself (should take
less than 5 minutes).
6. Remove the crucible cover and inspect your sample. If you see any blue-green crystals, reheat the sample
until the crystals have turned brown.
7. Measure and record the mass of the cool crucible of your copper chloride sample.
8. Transfer the brown solid to a clean and empty 150 mL beaker. Rinse out the crucible with three 8 mL
aliquots of distilled water and pour the solution into the 150 mL beaker. Gently swirl the beaker to
completely dissolve the solid. Note that the color of the solution is green as the copper ions are
rehydrated.
9. Measure out about 20 cm of aluminum wire, coil the wire (around a pencil…see example on Mr. Stein’s
desk), and place the wire in the beaker of solution so that it is completely immersed in the copper
chloride solution. Note that the reaction produces a gas, elemental copper is forming on the surface of
the aluminum wire, and the color of the solution is fading. The reaction will take about 30 minutes to
complete however we will leave it overnight as this is a good stopping point. Place a piece of clear wrap
over the top of your beaker to help prevent evaporation overnight. Clean up your station and place your
beaker in the marked area on the side wall (label your beaker so you know it is yours!)
Day 2:
10. When the reaction is done, the solution will be colorless. Most of the elemental copper will be on the
aluminum wire. Add 5 drops of 6 M HCl solution to dissolve any insoluble aluminum salts in the mixture,
which should make the solution clear. CAUTION: Handle the hydrochloric acid with care. It can cause
painful burns if it comes in contact with the skin.
11. Use a glass stirring rod, scoopula, or forceps to scrape off as much copper as possible into the clear
solution from the Al wire. Slide the wire up the wall of the beaker and out of the solution with the glass
stirrer and rinse off any remaining copper with distilled water. If any of the copper refuses to wash off the
aluminum wire, wash it with one or two drops of 6 M HCl solution. Put the Al wire aside.
12. To collect and wash the copper produced in the reaction:
a. Obtain a piece of filter paper and record its mass on your data table. Fold it properly and place it
in your funnel.
b. Use 3 mL amounts of distilled water to wash all of the copper onto the filter paper in the funnel
(do this 2 times). Use the glass stirring rod to break up the larger pieces of copper.
c. Wash the copper twice more with small amounts of distilled water.
13. Add 5 mL of 95% ethanol to the copper on the filter paper (allow it to filter through) and then let it sit for
about 1 minute.
14. Weigh a clean, dry watch glass and record its mass. Remove the filter paper from the funnel (with the
copper still in/on it) and place the paper and copper onto the watch glass. Spread it out on the watch
glass so evaporation of the liquid will happen more thoroughly. Put the watch glass with the filter paper
and copper over on the side of the room where you kept your beakers last night. Make sure to write on
the square you place your watch glass onto so you know which one is yours.
Day 3:
15. Get your watch glass and take the mass of the watch glass (with the filter paper and copper still on it).
Record this on your data table.
16. Dispose of the filter paper and copper into the labeled box in the fume hood. Clean your watch glass and
place it on the center lab station. Wipe down your station and return to your seat.
DATA ANALYSIS
1. How many moles of water were in your sample of copper chloride hydrate?
2. How many moles of copper were in your sample of copper chloride?
3. How many moles of chlorine were in your sample of copper chloride?
4. Write the proper chemical formula for the compound that you tested.
5. Why did we put the lids on the crucibles during the cooling process in Step 5?
6. Why was the solution clear after you left it overnight? What was responsible for the blue color?
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