Name:
Date:
Chemistry Experiment 4
Law of Definite Composition
Introduction:
The way in which compounds are formed has been shown to obey certain laws. One of these
laws is the Law of Conservation of Mass, which we have studied in class. Another law obeyed by
these chemicals is the Law of Definite Composition (also called the Law of Definite Proportions). The
Law of Definite Composition states that the elements that form a compound always combine in the
same proportions by mass. The compound water, H2O, is always a chemical combination of
hydrogen and oxygen in a 1:8 ratio by mass. If a mixture of hydrogen and oxygen were reacted in
the ratio 1:4 there would be water formed but there would also be some unreacted (left over)
hydrogen.
In this experiment you will examine the reaction between magnesium metal (Mg) and
oxygen gas (O2) in the air. The magnesium will be heated in a crucible and allowed to react with the
oxygen forming the compound magnesium oxide (MgO).
Purpose:
1. To observe the reaction between magnesium and oxygen.
2. To determine the ratio; mass of magnesium to mass of oxygen.
3. To verify the Law of Definite Composition.
Materials:
Crucible and Lid
Pipestem Triangle
Wire Mat
Analytical Balance
Safety:
Magnesium Ribbon
Tongs
Ring Stand and Rin
Burner
Steel Wool
Safety Goggles
Distilled Water
1. A magnesium fire is extremely difficult to put out. It will continue to burn even in the
carbon dioxide from a CO2 fire extinguisher.
2. Magnesium oxide (white smoke) is hazardous. Do not breathe in the smoke if escapes
out of the crucible.
3. You may be allergic to magnesium metal so after you clean the metal with the steel
wool, wash your hands thoroughly. Do not rub your eyes with magnesium metal on
your hands.
4. Safety goggles are essential for this experiment. Even if you're done your experiment
your goggles must be on until everyone is done.
Procedure:
1. Prepare the following summary in your data section.
Mass of crucible, cover and magnesium
Mass of crucible and cover
Mass of magnesium
_______________________
_______________________
_______________________
Mass of crucible, cover and product
Mass of product (Magnesium Oxide)
_______________________
_______________________
Name:
Date:
Mass of Oxygen reacted
_______________________
2. Clean the surface of the magnesium ribbon with a small portion of steel
wool by folding the steel wool around the magnesium and pulling the
magnesium ribbon through the steel wool. Do NOT clean the magnesium
on the lab bench.
3. Mass the crucible with the cover on the analytical balance and record this
mass in the data section.
4. Roll the magnesium into a loose coil and place it into the crucible. Find the
mass of the crucible, cover and magnesium and record this mass in the
data section.
5. Set up the ring stand, ring, burner, and pipestem triangle as shown in
Figure 1. Place the crucible on the pipestem triangle. Begin heating the
crucible gradually with the lid completely on. Remove the heat if large
amounts of smoke comes out of the crucible.
6. After about five minutes heat the crucible to redness for an additional five
to ten minutes with the lid slightly off.
7. Next heat the crucible with the lid completely off for about five minutes.
8. Allow the crucible to cool on the wire mat.
9. Mass the crucible, cover and product and record this mass in the data
section.
10. Clean up and make sure to wash your hands thoroughly.
Questions:
1. Calculate the mass of:
a. magnesium used
b. magnesium oxide produced
c. oxygen used
and record these in the data section.
2. Calculate the ratio of magnesium to oxygen.
Figure 1
Mg:O
3. Using the ratio (Mg:O) of 1: 0.658 as the accepted value calculate your percent error.
Error!
4. Use the accepted ratio to find the mass of magnesium that would be needed to combine with
16.0 grams of oxygen.
5. What mass of magnesium oxide would be formed in question 4?
6. Calculate the ratio of magnesium to magnesium oxide. (Use the data you obtained from the
lab)
7. What should be the ratio in question 6?
Sources of Error:
Conclusion: