Activity 2: Percent Composition of Magnesium Oxide


The Law of Definite Composition is introduced early in general chemistry as a part of exploring differences and similarities between compounds. Proust, a French chemist, was one of the first to state this principle. Using this principle, chemists in laboratories and industry are able to determine with high precision the amounts of each substance needed to make a compound.


This activity is designed to support the concept of a definite percent by mass of each element in the binary compound magnesium oxide.


  1. Wear protective goggles throughout the laboratory activity.

  2. Do not touch the hot crucibles. Hot porcelain and cold porcelain look the same.

  3. Follow proper procedures when lighting burners.

  4. Keep your hair pulled back away from your face so it will not come close to the burner flame. Hair ignites easily, especially if hair spray is used.

  5. Handle the Mg ribbon with tongs as much as possible. The ribbon is usually sharp along its edges from cleaning; it can cut easily. Do not look at the magnesium while it is burning.

  6. When handling crucibles, hold a piece of wire gauze up close under the crucible while moving the crucible from the clay triangle to the balance, etc. This will save many crucibles and prevent hot porcelain from shattering and causing burns.

  7. Dispose of materials as your teacher directs.

  8. Thoroughly wash your hands before leaving the laboratory.


1. Heat an empty crucible (with cover). Begin heating with a cool flame and gradually increase the temperature. Heat with a hot flame for 2-3 min. Allow to cool enough to handle with tongs. Weigh and record. Be careful not to place the crucible on counter tops, etc. that may melt or burn.

2. Place a coil of cleaned Mg ribbon provided for you in the crucible. (Your teacher may direct you to cut the Mg ribbon into small pieces to speed up the reaction.)

3. Weigh the crucible and Mg ribbon. Record.

4. Place the crucible and Mg ribbon in the clay triangle.

5. Heat the crucible and Mg ribbon in stages for 10-15 min. The clay triangle should glow red hot during this time.

6.Open the crucible with tongs to allow more air to enter while continuing to heat. Heat for 10 min more.

7. Remove the crucible and contents from the clay triangle and place it on a piece of wire gauze. Allow to cool about 5 min or until you can comfortably touch the crucible with a finger. Be Careful! Hold the back of your hand near the crucible to feel the heat.

8. Add 2 mL water to the crucible contents. (Use distilled water, if available.) Cover and return the crucible to the clay triangle.

9. Heat gently by moving the burner flame back and forth under the crucible for about 2 min. Then heat strongly for about 5 min to insure dryness.

10. Remove the crucible from the clay triangle and place it back on the wire gauze. Weigh when cool enough to handle. Record.

11. Thoroughly wash your hands before leaving the laboratory.

Data Analysis and Concept Development


Be sure to note colors of the Mg ribbon and the crucible contents during the activity. Also note any odor you may have encountered when water was added.

Anaylsis of Data

1. Using the mass of the contents (mostly MgO) and the mass of the magnesium ribbon you used, calculate the percent magnesium in magnesium oxide. Using the mass of oxygen and mass of contents, calculate the percent oxygen in magnesium oxide.

2. Assume the formula for magnesium oxide is MgO. Calculate the theoretical mass percent of Mg and O. Molar masses are:

3. Compare your data to theoretical values. Suggest reasons why your values might not match theoretical values.

4. Write a balanced equation for the primary reaction between Mg and O2.

Implications and Applications

  1. Why is it necessary to preheat the crucible?

  2. Why was it necessary to open the crucible lid to allow more air to enter during the reaction?

  3. Why was water added to the contents?

  4. What changes in the procedure could have been made, if time allowed, to insure that the Mg ribbon had completely reacted?

  5. How could you explain Mg mass percent values that are too high and those that are too low?

  6. Explain how you could use this general laboratory procedure to identify a compound if you know the elements that compose it.

  7. What effect would using unclean Mg ribbon have on your results?

  8. A student forgets to add water at the end of the procedure. What effect would this possibly have on the student's results? Do you think your equipment would be precise enough to detect this error? (Optional)