Laboratory Activity: Teacher Notes

Activity 2: Percent Composition of Magnesium Oxide

Major Chemical Concept

The activity illustrates the importance and usefulness of the Law of Definite Composition, emphasizes the mole concept, and involves calculations of empirical formulas.

Initial emphasis is on calculating percent composition; this is relatively easy for students to understand and calculations are straightforward. The determination of empirical formula and illustration of the utility of the mole concept are both clearer when pooled data for the entire class are used. Students calculate moles Mg and moles O from their experimental data. A plot of moles of Mg vs. moles of O for class data gives a graph with the expected slope of about 1.


General and honors. Also possibly basic, with enough teacher guidance.

Expected Student Background

Students should be able to calculate simple percent values and deal with molar mass. They should be familiar with standard laboratory procedures. Additional background is optional, such as applications of oxidation-reduction concepts to this laboratory activity.


This activity can be completed in one period if the pre-laboratory briefing is provided in a previous class period. If time becomes a concern, you can do the final weighing for students or save the final weighing until the next day, particularly if you have a drying oven to keep crucibles dry.


Read the Safety Considerations in the Student Version.

Materials (For 24 students working in pairs)



Advance Preparation

  1. All equipment should be readily available to students. Having needed equipment in plain view at laboratory stations saves considerable student work time. In general, check equipment ahead of time.
  2. Remove the oxide coating from the Mg ribbon by slightly sanding. Cut it in lengths with masses between 0.200 g and 0.400 g before the activity begins.
  3. Check the balances to insure they work properly.
  4. If distilled water is used, consider providing it in water bottles at each station.
  5. You may wish to pose some problems (e.g., percent composition, mole problems, etc.) for students to solve during the time samples are cooling.

Pre-Laboratory Discussion

  1. Demonstrate any procedural skills students have not seen before, such as proper handling of crucibles with tongs and the lighting of a burner. It is important to show the set up so students will be able to quickly assemble it themselves.
  2. Give students thorough instructions regarding safety. Warn students that cool and hot porcelain look the same; encourage caution when handling the crucibles. Holding a piece of wire gauze under the crucible when moving it from place to place will save some crucibles from rapid increases in entropy.
  3. Explain that under the conditions of this activity magnesium reacts with nitrogen gas in air as well as with oxygen gas. The contents should be as free as possible of magnesium nitride, Mg3N2 . This is accomplished by adding water to convert magnesium nitride into ammonia and magnesium oxide. A faint odor of ammonia, NH3 , may be detected by some students:

  4. Students should be asked about procedure details such as opening the lid [to allow more air in]. They should also be asked why it is necessary to heat the Mg ribbon strongly until the clay triangle and possibly the crucible itself glow red hot [to insure complete reaction and to dry the MgO after addition of water] .
  5. Insure that students understand how the collected data relate to the calculations they will perform.
  6. You may decide to supply problems for students to solve or provide some other activity to keep students busy during the time samples are cooling. If so, explain the nature of the assignment during the pre-laboratory orientation.