Magnesium is reacted with oxygen from the air in a crucible, and the masses before and after the oxidation are measured. The resulting masses are used to calculate the experimental empirical formula of magnesium oxide, which is then compared to the theoretical empirical formula. A crucible and Bunsen burner will be used to heat magnesium metal to burning.


A great deal of chemical knowledge has been amassed by using simple combustion experiments conducted with crucibles, burners, and balances. In this experiment, you are using this technique to experimentally determine the empirical formula of magnesium oxide.

This lab illustrates

  1. the law of conservation of mass and
  2. the law of constant composition.

Law of conservation of mass: the total mass of the products of a reaction must equal the total mass of the reactants.

Law of constant composition: Any portion of a compound will have the same ratio of masses as the elements in the compound.

Molecular composition can be expressed three ways:

  1. In terms of the number of each type of atom per molecule or per formula unit (the formula).
  2. In terms of the mass of each element per mole of compound.
  3. In terms of the mass of each element present to the total mass of the compound (mass percent).The empirical formula of a compound gives the lowest whole-number ratio of the constituent atoms that is consistent with the mass ratios measured by experiment.

In this lab, magnesium metal (an element) is oxidized by oxygen gas to magnesium oxide (a compound). Magnesium reacts vigorously when heated in the presence of air. The Mg-O2 reaction is energetic enough to allow some Mg to react with gaseous N2. Although there is a higher percentage of N2 gas in the atmosphere than O2, O2 is more reactive and the magnesium oxide forms in a greater amount than the nitride. The small amount of nitride that forms can be removed with the addition of water, which converts the nitride to magnesium hydroxide and ammonia gas. Heating the product again causes the loss of water and conversion of the hydroxide to the oxide.

The unbalanced equations are:

( 1 ) Mg(s) + N2(g) + O2(g) → MgO(s) + Mg3N2(s)

( 2 ) MgO(s) + Mg3N2(s) + H2O(l) → MgO(s) + Mg(OH)2(s) + NH3(g)

( 3 ) MgO(s) + Mg(OH)2(s) → MgxOy(s) + H2O(g)

Balancing the reactions are not necessary because the theoretical reaction product and yield is based on the amount of Mg available to react. The expected product is MgO, so the 1-to-1 mole ratio Mg to O in the product is all that is required.

Experimental Set Up

Heating a Substance in a crucible (1:31)

We are not doing this lab, but the general set-up is close to what you will expect.

Crucible Use

  • Crucibles are used to heat substances to high temperatures (like those encountered with burning metals) without risk of breakage. However, they are ceramic and can break. Please be careful — if your crucible breaks, please inform your teacher and get help with the clean-up and disposal. Please know that it is now chemical waste and must be placed in the solid waste container.
  • Do not touch the crucible with your hands (oils contaminate it and/or you could be severely burned).
  • Do not place a hot crucible on a lab bench (the temperature difference may cause it to break). Use the clay triangle or heating pad.

Prior to Starting

    • Practice using the tongs to pick up the lid from the crucible and the crucible from the clay triangle.
    • Practice placing the lid partially over the crucible so that there is a gap of about 0.5 cm (the lid should rest on the crucible edge and two legs of the triangle).
    • Practice placing the crucible with lid on the clay tile (when carrying the crucible, always hold it with tongs and support it with the tile).


Read and review carefully the procedure handout attached at the very bottom of this page. You may be tested with questions about the procedure.

Source: The content from this page was adapted from WebAssign General Chemistry Lab page. These labs were developed over many years by the instructional staff in the Department of Chemistry at the University of California, Santa Cruz. Many individuals have contributed to the efforts. Continuing support from the Heads of the Department during this period is acknowledged and appreciated.