Law of Definite Proportions
Objective: To measure and calculate the ratio of magnesium to oxygen in magnesium
oxide. To compare the lab ratio to the percent composition calculation based on the
formula.
Background
The Law of Definite Composition states that the elements that form a compound
always combine in the same proportion by mass. The compound water H20 always is a
chemical combination of hydrogen and oxygen in a 1:8 ratio by mass. If a mixture of
hydrogen and oxygen in some other ratio, say 1:2, were reacted, there would be water
formed, but there would also be some unreacted hydrogen, because water always forms
in the 1:8 ratio by mass.
In this experiment, you will examine the reaction between magnesium metal, Mg,
and the oxygen in the air, O2. The magnesium will be heated strongly in a crucible for
several minutes. The mass of magnesium will be compared with the mass of the material
produced.
Pre-lab Questions:
1. Why is it important to begin this experiment with a clean and dry crucible?
2. What is the purpose of making sure the outside of the magnesium ribbon is shiny?
3. With what element does the magnesium combine when it is heated?
4. Why must you reheat the crucible repeatedly until the last two masses agree
within 0.03g?
5. If you were to combine 80g of oxygen with some hydrogen, how much hydrogen
would you need to completely use up all the oxygen?
6. Suppose a compound of sodium and chlorine is formed in the ratio of 1.54g of
chlorine for each gram of sodium. How much sodium would you need to
completely react with 45.0g of chlorine?
Materials:
Crucible and Lid
Wire Gauze w/ Ceramic Center
Crucible Tongs
Safety Goggles
Centigram Balance Bunsen Burner and Striker
Ring Stand and Ring
Magnesium Ribbon
Distilled Water Bottle
Pipe Stem Triangle
Procedure:
1. Put on your goggles. Hot crucibles and magnesium can cause burns, so use with
caution. Handle hot crucibles with tongs and place the hot crucible on the wire
gauze to cool.
2. Obtain a clean and completely dry crucible and cover. Find the mass of the
crucible and cover and record it on the data table.
3. Obtain a piece of magnesium from your instructor and roll the magnesium into a
loose coil and place it in the crucible. Find the mass of the crucible, cover, and
magnesium. Record it on the data table.
4. Set up the ring stand, ring, burner, and pipe stem triangle. Place the crucible on
the pipe stem triangle. Begin heating the crucible gradually with the lid
completely on. Heat slowly by moving the flame around underneath the crucible.
Remove the heat temporarily if a large amount of smoke comes out of the
crucible.
5. After about four minutes of direct heating with no smoke, remove the lid slightly.
Heat the crucible for four more minutes. Finally, remove the lid completely and
heat strongly for four more minutes.
6. Turn off the burner and put the lid back on the crucible. Allow the crucible and
cover to cool to a temperature low enough so that you can touch the crucible. Find
the mass of the crucible, contents, and cover. Add ten drops of distilled water.
Smell cautiously, note any odor. Put the crucible back on the ring-stand setup and
heat again for four minutes with the lid on. Allow to cool again.
7. Find the mass of the crucible, cover, and product. Record it on the Report Sheet.
8. If enough time remains, reheat the crucible for four minutes, allow it to cool, and
again find the mass. If this mass differs by more than 0.03g from the mass you
found in Step 8, repeat this procedure for a second trial.
9. If enough time remains, repeat the whole procedure for a second trial.
10. Clean and put away all of the materials.
11. Wash your hands thoroughly with soap and water.
Data and Observations:
Mass of Crucible and Cover
Mass of crucible, cover, & Mg
Mass of crucible, cover, & product (1st)
Mass of crucible, cover, & product (2nd)
Mass of crucible, cover, & product (3rd)
Calculations: SHOW WORK
1. Calculate the mass of the Mg that reacted.
2. Calculate the mass of the magnesium oxide that was produced.
3. Calculate the mass of oxygen that reacted.
4. Calculate the ratio of the mass of magnesium to the mass of oxygen.
5. The accepted ratio for the mass of magnesium to oxygen is 1.52:1. Calculate your
percent error.
Mass of Magnesium reacted.
Mass of Magnesium Oxide produced.
Mass of Oxygen reacted.
Ratio of Magnesium to Oxygen.
Percent Error
Post-Lab Discussion:
The magnesium metal is an element that combines with another element, oxygen gas, to
form the compound magnesium oxide. The ratio of the mass of magnesium oxide to the
mass of magnesium should be constant for all of your trials, regardless of the mass of the
magnesium that you started with.
The strong heating insured that all of the magnesium reacted with the oxygen in
the air to form magnesium oxide. Since some magnesium nitride (magnesium + nitrogen)
could have formed, the addition of water and subsequent heating were done to remove
that product from the crucible.
In order to calculate the ratio, you must first find the masses of magnesium oxide
alone and of magnesium alone by subtracting the mass of the crucible from the masses
that you recorded. The ratio is then calculated by:
Ratio = Mass of Magnesium Oxide / Mass of Magnesium
Analysis:
1. How would your results be affected if all of the magnesium did not react?
2. Using your ratio, determine the formula of magnesium oxide.
3. Use the accepted ratio to determine the mass of magnesium that would combine
with exactly 16.0g of oxygen.
4. Using the accepted ratio of magnesium to oxygen, determine the formula mass of
one mole of magnesium oxide.
5. Calculate the percent composition of magnesium and oxygen in one mole of
magnesium oxide.
6. Suppose you tried to combine 42g of Mg with 45g of oxygen.
a. Which of the substances would have some left after the reaction?
b. How much magnesium oxide would be formed?
Conclusion:
When 1.0lb of gasoline is burned in an automobile, approximately 3lbs of carbon dioxide
is given off. Carbon dioxide is one of the gases contributing to global warming. What
information from this experiment helps to explain how one pound of gasoline can give
off approximately 3 times as much CO2?