Laredo Community College
Science Department CHEM 1411 Laboratory 53
CHEMICAL REACTIONS: EMPIRICAL FORMULA
A. Short Overview
Review Sections 3.4 - 3.6 in your textbook (Chemistry: The Central Science, 9th Ed., Brown, LeMay,
Bursten & Burdge) on ionic compounds and empirical formulas.
In the earlier experiment on physical and chemical properties, you learned that chemical reactions occur
when a substance undergoes a chemical change, and the properties of the products are different from those
of a the reactants. This experiment expands on the chemical changes by allowing you to determine
quantitatively the amounts of reactants involved in making a product, and represents an introduction to the
law of definite proportions in describing how chemical formulas of compounds are determined.
The particular product of the reaction is an ionic compound, the combination of a metal (magnesium) with
a nonmetal (oxygen). Because the product that is formed is ionic, we cannot describe the compound as
containing discrete molecules; rather, the product contains charged ions which are held together by strong
electrostatic attractions. Overall, the compound is electrically neutral.
The primary purpose of this experiment is to determine the ratio of magnesium ions to oxide ions in the
formula. This ratio determines the empirical formula of the compound, which we shall represent by the
A secondary purpose of the experiment is to introduce you to the concept of gravimetric analysis in the
laboratory, and the method of obtaining usable data.
The chemical equation associated with the reaction is
x Mg (s) + y/2 O2 (g) MgxOy (s)
You will measure the mass of oxygen by the process of weight-by-difference, which you have already used.
Then, by using the experimental mass and the molar mass of both magnesium and oxygen, you will be able
to determine a molar ratio. The simplest integer ratio of moles of magnesium to moles of oxygen represents
the empirical formula for the compound.
Since the reaction is carried out in air, which is a mixture of 78% nitrogen and 21% oxygen (as well as other
inert elements and compounds), the initial product will be a mixture of magnesium nitride and magnesium
oxide. The magnesium nitride formed during the reaction can be converted to magnesium hydroxide and
ammonia by the addition of water, and the hydroxide can be converted to the oxide by dehydration. These
3 Mg (s) + N2 (g) Mg3N2 (s)
Mg3N2 (s) + 6 H2O () Mg(OH)2 (s) + 2 NH3 (g)
Mg(OH)2 (s) MgO (s) + H2O ()
CHEM 1411 Laboratory Empirical Formula 54
Exercise 1. Empirical Formula of Magnesium Oxide
Chemicals: Magnesium (Mg), Propane (C3H8), 3 M HNO3 (aq)
Apparatus: crucibles & tops, electronic balances, tripod, wire gauze, wire triangles, Bunsen burner,
sandpaper, wood splint
Safety Equipment: goggles, gloves, hood.
Objectives: In this experiment you will learn how to:
1. heat a sample in a crucible with a Bunsen burner.
2. use the method of “weight-by-difference” to determine mass quantities.
3. convert grams to moles.
4. determine the simplest ratio between two molar quantities, and to convert the ratio into a chemical
Each student will heat one sample. Determine a method for identifying your crucible and cover, and not
getting them mixed up with your partner(s) crucible & top.
On the data sheet, the data for your sample is placed in column listed “Trial 1”. Your partner’s data is listed
in column(s) “Trial 2” (and “Trial 3” if a group of three).
Use the same electronic balance for each weighing. Tare the balance each time to 0.000 g before placing the
crucible and cover. Be sure to include the cover with the crucible each time.
CAUTION: Tie your hair behind your head.
1. Obtain a bunsen burner and tripod from the front of the lab. Place a wire triangle from your desk on
top of the tripod.
2. Clean a crucible and cover with soap and water, rinse with distilled water, and dry the crucible with
3. Use your crucible tongs to place the crucible and lid on the triangle. Do not cover the crucible with
the lid, but place it slightly off-center.
4. Connect a bunsen burner to the propane gas line with rubber hose. (CAUTION: Check the burner
hoses carefully before lighting. Replace the burner hoses if they are old or cracked.) Turn on the
gas and light the burner. Adjust the flame so that it is pale blue and does not appear yellow.
(Yellow flames are caused by incomplete combustion and will leave soot on the crucible; yellow
flames are also cooler than blue flames.) Place the burner under the crucible.
5. Heat the crucible until it is red-hot for about 5 minutes. Use the tongs to place the crucible and top
on the desk, and allow it to cool to room temperature. The crucible and cover will get very hot
when heated. Do not touch the hot crucibles or covers with bare hands. Allow the crucibles to cool
to room temperatures before weighing. The crucible is cool enough to weigh when it feels cool as
you pass your hand over it.
CHEM 1411 Laboratory Empirical Formula 55
Do not try to cool down the hot crucibles or covers by running cold water over them; the porcelain
crucibles will breaks. Also, do not set the hot crucibles on your lab notebooks; they will burn
through the notebook paper.
6. Weigh the crucible and cover on the electronic balance to the nearest 0.001 g.
7. Obtain a loop of magnesium metal from the chemicals cart. Unroll the loop and cut it into five or
six smaller pieces, and transfer the magnesium to the crucible. Reweigh the crucible on the same
balance to ± 0.001 g. Determine the mass of the magnesium. It should be between 0.25-0.35 gram
in order to obtain the best results. Add another small piece of magnesium if you do not have enough
NOTE: The next time you will weigh the crucible is in step 10.
8. Place the crucible and cover in the triangle, with the lid slightly off-center as in step 3. Heat the
crucible gently by passing the flame under the bottom of the crucible until the magnesium burns
with the bright white flame. (CAUTION: Magnesium burns with a bright, white flame similar to
a welder’s torch or sparkler. Do not look directly at the magnesium when it burns.)
NOTE: If the magnesium strip has not started to burn after 10 minutes of heating, light a
wood splint and hold it inside the crucible until the magnesium ignites.
9. Once the magnesium has burned totally, heat the crucible strongly at red heat for 10 minutes. Cool
the crucible to room temperature, then add 10-15 drops of distilled water to the crucible to convert
the magnesium nitride to magnesium oxide. Evaporate the water using a low flame, so as to not
lose any sample through splattering.
10. Cool the crucible to room temperature and weigh the crucible and lid. Then reheat with a strong
flame for another five minutes, cool and reweigh. Repeat the heating process for three minutes, and
rewigh a third time. The weights after the two final heatings should agree within 0.005 g. If not,
repeat the heating process for three additional minutes, followed by cooling and weighing.
11. Carefully add 1-2 mL of 3 M HNO3 (aq) to the crucible to dissolve the solid residue. Rinse the
crucible with water and discard the solution in the sink.
12. Return all borrowed equipment to the equipment cart at the front of the lab.
CAUTION: Be sure to turn the propane gas off when you finish.
There are no special disposal instructions for the experiment. All solutions may be discarded in the sink.
CHEM 1411 Laboratory Empirical Formula 56
1. Determine the moles of oxygen in the sample by dividing the mass of oxygen by 16.00.
2. Determine the moles of magnesium by dividing the mass of magnesium by 24.30.
3. Determine the ratio of the moles of oxygen to moles of magnesium by
4. Determine the simplest whole number ratio for the compound.
5. Determine the % Mg in the sample.
6. Determine the % error for the experiment, using MgO as the accepted formula.
Table 1. Decimal Fractions of 1 – 10
1 2 3 4 5 6 7 8 9 10
1 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
2 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000
3 0.333 0.667 1.000 1.333 1.667 2.000 2.333 2.667 3.000 3.333
4 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500
5 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800 2.000
6 0.167 0.333 0.500 0.667 0.833 1.000 1.167 1.333 1.500 1.667
7 0.143 0.286 0.429 0.571 0.714 0.857 1.000 1.143 1.286 1.429
8 0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000 1.125 1.250
9 0.111 0.222 0.333 0.444 0.556 0.667 0.778 0.889 1.000 1.111
10 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000
CHEM 1411 Laboratory Empirical Formula 57
E. Lab Report
Open the spreadsheet "Empirical Formulas" on the CHEM 1411 website. The URL for the website is
When you open the website, click on the link for "CHEM 1411". You will then click on the icon for the
Enter your data in the cells WITH BORDERS only. As you enter your values, the program will
automatically calculate the masses, moles, and % composition.. When you finish, print out the data and
include in your lab report.
A sample spreadsheet is shown below.
Trial 1 Trial 2 Trial 3
Mass of empty crucible & cover 18.245 21.882 18.625 g
Mass of crucible, cover, and Mg 18.563 22.210 18.872 g
Mass of Mg 0.318 0.328 0.247 g
Moles of Mg 0.01309 0.01350 0.01016 mol
Mass of crucible & oxide, final
heating 18.743 22.408 18.994 g
Mass of magnesium oxide 0.498 0.526 0.369 g
Mass of oxygen 0.180 0.198 0.122 g
Moles of oxygen 0.01125 0.012375 0.007625 mol
Moles Mg ÷ Moles O 1.163 1.091 1.333
% Mg (experimental) 63.9% 62.4% 66.9% %
% Mg in MgO (theoretical) 60.3% 60.3% 60.3%
% error 5.9% 3.4% 11.0% %
CHEM 1411 Laboratory Empirical Formula 58
1. Determine the percent composition for the following empirical formulae: MgO; MgO2; Mg2O;
Mg2O3; Mg2O5; Mg3O2; Mg3O4
2. Determine the experimental percent composition for your samples.
3. Based on your answers to questions 1 & 2, describe how you can predict the formula for the oxide
from your experimental data.
4. Assuming that the true formula of the oxide is MgO, what is the percent error in your experiment?
CHEM 1411 Laboratory Empirical Formula 59
Empirical Formula of Magnesium Oxide
Trial 1 Trial 2 Trial 3
1. Mass of empty crucible & cover g g g
2. Mass of crucible, cover, and Mg g g g
3. Mass of Mg (2) – (1) g g g
4. Moles of Mg (3) 24.30 mol mol mol
Mass of crucible & oxide
5. 1st heating g g g
6. 2nd heating g g g
7. 3rd heating g g g
8. Mass of magnesium oxide (7) – (1) g g g
9. Mass of oxygen (8) – (3) g g g
10. Moles of oxygen (9) 16.00 mol mol mol
11. Moles Mg ÷ Moles O ______________ ______________ ______________
12. Empirical Formula, MgxOy ______________ ______________ ______________
13. % Mg (experimental) ______________% ______________% ______________%
14. % Mg in MgO (theoretical) ______________% ______________% ______________%
15. % error ______________% ______________% ______________%
Show all calculations on the back of the page.