Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Experiment 7 – Empirical Formula
Introduction
In 1794, a French chemist, Joseph Proust, published a paper detailing a set of experiment
designed to prove his hypothesis that when elements come together to form a compound, they do
so in small, whole number ratios. He compared naturally occurring copper (II) carbonate,
CuCO3, to artificially made CuCO3. Here, he varied the amount of copper, carbon, and oxygen
in multiple ways, but found that, by mass, the compound he made was identical to the naturally
occurring compound. Eventually other scientist began to confirm his findings, and the Law of
Constant Composition (sometimes called Law of Definite Proportions) was formed.
The empirical formula for a chemical compound is the simplest, whole number ratio of the
elements. For example, when sodium, Na, and chloride, Cl, come together, the familiar
compound, sodium chloride, NaCl, is formed. Even if more than one atom of each of the
elements is involved in making the compound, the ratio will always be 1:1. Similarly, if
calcium, Ca, reacts with chlorine, Cl, the compound calcium chloride, CaCl2 is formed, and it is
always in a 1:2 ratio.
This elemental ratio is determined by the mass of each of the elements reacting to form the
compound. Recall that Conservation of Mass requires that all mass of the reactants show up in
the formation of the products. In measuring how much of the reactants were used to form the
product, the number of moles of the reactants can be determined using the periodic table. These
numbers of moles are, in turn, the subscripts you see in the chemical formula of a compound (the
invisible 1 and 2 seen in CaCl2, above).
In the first part of this experiment, you will react magnesium metal with oxygen to form
magnesium oxide and determine its empirical formula. In the second part of the experiment,
copper will be reacted with sulfur to make copper sulfide and determine its formula.
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Examples
1) Lead is reacted with oxygen to form a lead oxide. If 136.752 g of lead reacted with oxygen
to form 158.512 g of a lead oxide, what is the empirical formula of this lead oxide?
The amount of lead is clearly stated as 136.752 g. In order to determine how much oxygen
reacted, use conservation of mass.
158.512 g product – 136.752 g lead = 21.760 g oxygen that must have reacted
1 𝑚𝑜𝑙 𝑃𝑏
136.752 𝑔 𝑃𝑏 [207.2 𝑔 𝑃𝑏] = 0.66 𝑚𝑜𝑙 𝑃𝑏
1 𝑚𝑜𝑙 𝑂
21.760 𝑔 𝑂 [ 16 𝑔 𝑂 ] = 1.36 𝑚𝑜𝑙 𝑂
This yields a molar ratio of the elements of: Pb0.66O1.36
However, this is not a whole number ratio. To achieve this, divide both subscripts by the
smaller of the two.
𝑃𝑏0.66 𝑂1.36 = 𝑃𝑏1 𝑂2.06
0.66
which is essentially PbO2
0.66
2) Aluminum is reacted with oxygen to form aluminum oxide. If 21.066 g of aluminum reacts
to form 39.626 g of aluminum oxide, what is the empirical formula of this aluminum oxide?
As before, the amount of oxygen used in making the compound is the difference between the
products and reactants.
39.626 g product – 21.066 g aluminum = 18.560 g oxygen that must have reacted
1 𝑚𝑜𝑙 𝑃𝑏
21.066 𝑔 𝐴𝑙 [26.98 𝑔 𝑃𝑏] = 0.781 𝑚𝑜𝑙 𝐴𝑙
1 𝑚𝑜𝑙 𝑂
18.560 𝑔 𝑂 [ 16 𝑔 𝑂 ] = 1.16 𝑚𝑜𝑙 𝑂
This yields a molar ratio of the elements of: Al0.781O1.16
However, this is not a whole number ratio. To achieve this, divide both subscripts by the
smaller of the two.
𝐴𝑙0.781 𝑂 1.16 = 𝐴𝑙1 𝑂1.49 which is still not a whole number, but if this entire
0.781
0.781
compound ratio is doubled, it results in Al2O3, which is a small, whole number ratio.
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Procedure
Magnesium and Oxygen
Place a crucible in a clay triangle and support it on the ring stand with an iron ring. Refer to
Figure 7.1 for the set up. Heat the crucible for 10 minutes on the burner to burn off any
contaminates that might be present, then allow the crucible to cool to room temperature.
Weigh the crucible and record its mass.
Obtain a piece of magnesium ribbon, coil it gently such that it generally covers the bottom of
the crucible, and place inside the crucible. Reweigh the crucible with the magnesium in it
and record the mass.
Place the crucible back on the clay triangle and begin to heat the crucible. When the
magnesium begins to spark and smoke, cover the crucible with a crucible lid. Once the
smoke has stopped billowing from the crucible, continue to heat the crucible, checking the
contents occasionally by carefully lifting the lid. The magnesium will turn white.
Once the material appears to have finished reacting, turn off the burner and allow the crucible
cool to room temperature. Use a pipet to place a few drops of water into the crucible. This
will cause the white residue to fizz.
Cover the crucible once again and reheat the crucible for 10 more minutes, then allow it to
cool to room temperature. Reweigh the crucible with the resulting product.
Clean the crucible and repeat the experiment.
Calculate the empirical formula for the product.
The waste magnesium oxide can be thrown away in the trash. Return the crucible to its place
of origin.
Safety Precaution: The crucible can look cool when it is not, be careful in checking for
whether or not it has cooled to room temperature. Use tongs as necessary.
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Copper and Sulfur
This section of the experiment must be performed in a fume hood!
Place a crucible in a clay triangle and support it on the ring stand with an iron ring. Refer to
Figure 7.1 for the set up. Heat the crucible for 10 minutes on the burner to burn off any
contaminates that might be present, then allow the crucible to cool to room temperature.
Weigh the crucible and record its mass.
Obtain a piece copper wire, coil it gently such that it generally covers the bottom of the
crucible, and place inside the crucible. Reweigh the crucible with the copper in it and record
the mass.
Completely cover the copper wire with sulfur, place the lid a bit askew on the crucible, and
heat vigorously for 10 minutes. Turn off the burner and allow the crucible to cool to room
temperature and weigh it.
Continue this process until two masses of the product are within 0.005 g of one another.
Clean the crucible and return it to its place of origin.
The waste copper sulfide can be thrown away in the trash.
Figure 7.1: Experimental Set Up and
magnesium in the crucible
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Prelaboratory Questions
1) What is an empirical formula?
2) Why is the initial firing of the crucible essential?
3) When is the magnesium done reacting?
4) When is the copper done reacting?
5) A 0.757 g sample of calcium metal is placed in a 40.400 g crucible, covered in sulfur, and
heated to react it. If the resulting product weighed 41.959 g, what is the empirical formula for
the product.
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Data Table
Magnesium and Oxygen
Trial 1
Trial 2
__________ g
__________ g
Mass of crucible
__________ g
__________ g
Mass of Mg
__________ g
__________ g
__________ g
__________ g
__________ g
__________ g
Mass of crucible + Mg
(before heating)
Mass of crucible + product
(after heating)
Mass of oxygen that reacted (after – before heating)
Calculate the empirical formula for the magnesium oxide – show calculations
Empirical Formula for magnesium oxide
__________
__________
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Copper and Sulfur
Mass of crucible + Cu (before heating)
__________ g
Mass of crucible
__________ g
Mass of Cu
__________ g
Mass of crucible + product
(1st heating)
__________ g
Mass of crucible + product
(2nd heating)
__________ g
Mass of crucible + product
(3rd heating)
__________ g
Mass of crucible + product
(4th heating)
__________ g
Mass of crucible + product
(5th heating)
__________ g
Calculate the empirical formula for the copper sulfide – show calculations
Empirical Formula for copper sulfide
_________
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
Postlaboratory Questions
1) Sodium reacts with chlorine to form NaCl. Calcium reacts with oxygen to form CaO.
Potassium reacts with nitrogen to form K3N. Lithium reacts with oxygen to form Li2O.
Magnesium reacts with fluorine to form MgF2. Calcium reacts with nitrogen to form Ca3N2.
Given these six compounds, predict how the following elements would come together and write
their chemical formulas.
Mg + S
__________
Ca + P
__________
Li + O
__________
K+F
__________
Mg + I
__________
Na + N
__________
Ca + S
__________
K+O
__________
Li + N
__________
Li + P
__________
Sr + Br
__________
Ba + O
__________
2) Calcium metal is allowed to react with oxygen. If 12.823 g of calcium reacted with 5.121 g
of oxygen, what is the empirical formula for the product?
3) A 43.641 g sample of phosphorous is burned in oxygen. If the resulting product weighs
100.021 g, what is the empirical formula for the compound?
Experiment 7 – Empirical Formula
Name __________________
Lab Section __________________
4) Pyrite, commonly called Fool’s Gold, is a compound of iron and sulfur. If 11.393 g of iron
reacts with excess sulfur to form 24.542 g of Fool’s Gold, what is the empirical formula for this
compound?
5) Caffeine, the stimulant found in coffee, tea, chocolate, and sodas, contains carbon, hydrogen,
nitrogen, and oxygen. If a 100.0 g sample of caffeine has 49.48 g carbon, 5.15 g hydrogen,
28.87 g nitrogen, and 16.49 g oxygen, what is its empirical formula? Given that its molar mass
is 194.2 g/mol, what is its true molecular formula? (hint: see how many of your calculated
empirical formulas would add up to make the true mass of caffeine)