Experiment 7: Determination of a Chemical Formula
Introduction
Molar Ratios
Elements combine in fixed ratios to form compounds. For example, consider the compound
TiCl4 (titanium chloride). Each molecule of TiCl4 is composed of 1 titanium and 4 chlorine
atoms. Therefore, 1 mole of titanium will combine with 4 moles of chlorine to produce 1 mole of
TiCl4. The molar ratio of titanium to chlorine in the compound is 1 to 4. This can be expressed
as a fraction:
moles of Ti in compound
1
--------------------------------- = ----- = 0.25
moles of Cl in compound
4
Alternatively, we can think about the molar ratio of chlorine to titanium in TiCl4, which is 4 to 1.
This can also be expressed as a fraction:
moles of Cl in compound
4
---------------------------------- = ----- = 4.0
moles of Ti in compound
1
Suppose you don’t know the formula of a particular compound. If you know the molar ratio of
two elements in the compound, you can use that ratio to determine the simplest possible formula
for the compound. For example, suppose you determined the following ratio in the lab for a
compound that is composed of nitrogen and hydrogen:
moles of H in a sample of the compound
0.0299 mol
3
------------------------------------------------------- = ----------------- = 2.99 ≈ ---moles of N in a sample of the compound
0.0100 mol
1
This data could also be expressed as follows, with the moles of N over moles of H instead:
moles of N in a sample of the compound
0.0100 mol
1
------------------------------------------------------- = ----------------- = 0.334 ≈ --moles of H in a sample of the compound
0.0299 mol
3
From either of these ratios (H over N, or N over H) you know that there are approximately 3 H’s
for every 1 N in the compound. Therefore, the simplest possible formula for the compound is
NH3.
In this experiment you will determine the chemical formula of a compound that is composed of
only copper (Cu) and chlorine (Cl). You will not know the formula of this compound ahead of
time, but you will be able to determine its formula by determining the number of moles of copper
that combine with a given number of moles of chlorine. You will calculate the moles or copper
and chlorine from the masses (grams) of copper and chlorine in the compound, which are easily
measured in the lab. The prelab sheet provides an example of exactly how this will be done, but
uses a compound containing hydrogen and oxygen instead of copper and chlorine.
CHEM100L-Expt 7
2
Determination of the Chemical Formula of a Compound Containing Copper and Chlorine
You will be given a sample of a compound that is known to be composed only of copper and
chlorine. More specifically, it is composed of copper ions and chloride ions. To determine the
formula of the compound, the first step will be to determine the number of grams of copper and
grams of chlorine in the sample of the compound.
To determine the masses (in grams) of copper and chlorine in the sample, you will need to
separate the copper from the chlorine. This separation will be accomplished by carrying out a
chemical reaction in which all of the copper ions in the compound undergo a reaction in which
they are converted to solid copper metal. The copper metal can then be filtered out of solution
and its mass determined.
Below is the reaction that will be carried out to convert the copper ions to copper metal (the
reaction is not balanced):
Cu ions
from
compound
+
Cl- +
from
compound
Al metal
aluminum
foil

Cu metal
+
will appear
as brown particles
Cl-
+
Al ions
For the reaction, the compound is dissolved in water and mixed with aluminum metal (pieces of
aluminum foil). The copper ions from the compound react with the aluminum foil. Notice that
the copper ions are converted to solid copper metal, and the aluminum metal is converted to
aluminum ions. Also notice that the chloride ions from the compound do not react—they remain
unchanged.
After the reaction is complete, the brown particles of copper metal that have formed can be
filtered out of the solution. The particles will then be dried and their mass determined. Based on
that mass, you will be able to determine the formula of the compound, using the calculation steps
outlined on the prelab.
Conversion of Mass to Moles
If you do not remember how to convert a mass (in grams) to moles, use the example below as a
guide.
Convert 5.000 grams of zinc to moles of zinc. The molar mass of zinc (65.38 g/mol) can
be obtained from the periodic table.
5.000 g of zinc
1 mole of zinc
-------------------------------------------- = 0.07648 moles of zinc
65.38 g
Therefore, 5.000 g of zinc is equivalent to 0.07648 moles of zinc.
CHEM100L-Expt 7
Name: _________________________________
Experiment 7: Determination of a Chemical Formula
Pre-Lab Exercise
1. a) Suppose a certain compound contains only hydrogen and oxygen. Analysis of a 5.00 g
sample of this compound revealed that the sample contained 4.44 g of oxygen. How many
grams of hydrogen does the sample contain? Show your work here.
b) Convert the grams of oxygen (4.44 g) to moles of oxygen. Show your work below and
include units on each number in the calculation. (From the periodic table, oxygen’s molar
mass is 16 g/mol)
c) Convert the grams of hydrogen (that you calculated first) to moles of hydrogen. Show your
work below and include units on each number in the calculation. (From the periodic table,
hydrogen’s molar mass is 1 g/mol)
d) Now that you know the number of moles of oxygen (O) and the number of moles of
hydrogen (H) the 5.00 g sample contained, what is the H to O molar ratio of this compound?
e) Based on this H to O molar ratio, what is the simplest formula of this compound?
In today’s experiment you will begin with a compound containing only copper and chlorine,
and determine the grams of copper in a sample of the compound. Then you will do the same
calculations as above in order to determine the simplest formula for the copper/chlorine
compound.
CHEM100L-Expt 7
4
Experiment 7: Determination of a Chemical Formula
Procedure
1. Weigh out approximately 2.5 g of the copper chloride compound and record the exact mass
(all digits from the reading on the balance) on the report sheet. Place it in a 150 mL beaker and
add 40 mL of purified water. Stir until the compound is completely dissolved—be patient as this
may take a few minutes.
2. To the beaker add approximately 0.5 g of aluminum foil which has been cut into small
squares. Use a stirring rod to push the foil into the solution if necessary. The reaction should
begin to occur immediately and should be obvious. Wait until the reaction ceases and the
solution becomes less cloudy (~10 minutes; when most of the bubbling has stopped). While
waiting, obtain a Buchner funnel and filter flask from the stockroom.
3. Add 10 mL of 6 M HCl to dissolve the excess aluminum foil that remains. Wait until this
reaction ceases, at which point there should be no tiny particles of aluminum remaining. If you
can still see particles of aluminum in the solution, ask the instructor for help. You should be able
to see a large quantity of brown solid that has precipitated to the bottom of the beaker—this is
the copper metal (in very fine particles).
4. Obtain 30 mL of acetone in a beaker and set it aside for the next step. Fill your wash bottle
with distilled water and set it aside. Place the Buchner funnel into the top of the filter flask, and
clamp the neck of the flask to a ring stand so that it doesn’t tip over. Then set up a vacuum
filtration apparatus by connecting the arm of the filter flask to a water aspirator using one of the
heavy hoses from the cabinet under your lab bench. Place a piece of filter paper into the
Buchner funnel, making sure that all the holes are covered. Wet the filter paper so that it “sticks”
in place by squirting it with distilled water from your wash bottle. Ask the instructor to check
your setup before proceeding with vacuum filtration.
5. Turn on the water aspirator to create a vacuum in the flask—this will draw liquid from the
Buchner funnel into the flask, and filter out any solid particles in the process. Pour your reaction
mixture into the Buchner funnel in order to filter out the solid copper. You must try to get ALL
copper particles into the Buchner funnel. If any copper remains stuck to the sides of the beaker,
use as much distilled water from your wash bottle as necessary to wash the particles into the
funnel. Wash the copper in the Buchner funnel with three additional portions of distilled water
(~10 mL each) in order to rinse away any chloride or aluminum ions from the reaction mixture
that may remain stuck to the copper.
CHEM100L-Expt 7
5
6. Break the vacuum by detaching the hose from the arm of the filter flask. Pour about 1/3 of
the acetone into the Buchner funnel to cover the copper; allow it to drip through by gravity.
Acetone will wash much of the water away from the copper, speeding up the subsequent process
of drying the copper. Water evaporates slowly, but acetone evaporates much more quickly.
Therefore, you will only have to wait a short period of time for the acetone to evaporate before
weighing the dry copper.
When all the acetone has dripped out of the Buchner funnel, pour another portion of the
remaining acetone into the funnel to cover the copper, and allow it to drip through. Then repeat
this process one more time with the last of the acetone.
When the last portion of acetone has dripped out of the funnel, re-attach the hose to the arm of
the filter flask (be sure the aspirator is turned on to create a vacuum). Allow the vacuum to draw
air through the copper in the funnel for a few minutes in order to help the acetone evaporate.
7. Detach the hose from the arm of the filter flask and carefully remove the Buchner funnel from
the top of the flask. Transfer the copper to a clean, dry evaporating dish, being careful not to
lose any particles of copper in the process. Scrape as much copper as possible off of the filter
paper and into the evaporating dish. The filter paper can then be discarded. Label the dish with
your name by attaching a piece of tape to the edge. Put the dish in the oven for 15 minutes to dry
the copper thoroughly (the oven should be on setting 5, at 110 oC).
8. The liquid in the filter flask can be discarded down the drain. Wash the Buchner funnel and
flask and return them to the stock room. Check the copper after it has been in the oven for 15
minutes. When it is dry, it should seem powdery, rather than sticky, when stirred with a glass
stirring rod.
9. Allow the dry copper to cool. Then weigh it and record its exact mass on the report sheet.
Ask the instructor how to dispose of the copper. Complete the rest of the calculations on the
report sheet in order to determine the formula of the compound.
CHEM100L-Expt 7
Name: __________________________
Lab Partner: __________________________
Desk: _____________ Date: _____________
Experiment 7: Determination of a Chemical Formula
Report Sheet
Data
1. Mass of original sample of compound
__________ g
2. Mass of copper obtained
__________ g
Calculations
3. Mass of chlorine in original sample
Show work here:
__________ g
4. Moles of copper obtained
Show work here:
__________ mol
5. Moles of chlorine in original sample
Show work here:
__________ mol
6. Molar ratio of chlorine to copper:
Record the exact number from the calculation.
Show work here:
___________ moles of Cl / 1 mole of Cu
7. Simplest formula for the compound:
____________
CHEM100L-Expt 7
7
Experiment 7: Determination of a Chemical Formula
Post-Lab Exercise
1.
Circle one answer for each of the following:
If you did not let the copper dry thoroughly, how will the apparent mass of copper (line #2)
be affected? too high or too low
How will the calculated mass of chlorine (line #3) be affected? too high or too low
How will the calculated moles of copper (line #4) be affected? too high or too low
How will the calculated moles of chlorine (line #5) be affected? too high or too low
How will mole ratio of chlorine to copper (line #6) be affected?
too many moles of Cl for one mole of Cu or too few moles of Cl for one mole of Cu
Finally, based on the answers you circled above, will the formula (line #7) come out correctly for
the compound even if you failed to let all the water evaporate from the copper? YES or NO
2.
What was the purpose of adding 6 M HCl in step 3 of the procedure?
3.
What was the purpose of washing the copper residue with acetone?
If you skipped this step with HCl, how would the apparent mass of copper (line #2)
be affected? too high or too low
How would the calculated mass of chlorine (line #3) be affected? too high or too low
CHEM100L-Expt 7