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Experiment 11 -Determination of the Empirical Formula of
Magnesium Oxide
When magnesium and oxygen are heated together, they readily undergo a
chemical change (reaction):
magnesium + oxygen magnesium oxide (Rxn.1)
From the masses of magnesium and oxygen that combine, we can calculate the empirical
formula of magnesium oxide. We will weigh the magnesium before it combines with the
oxygen, and we will also weigh the product of the reaction, magnesium oxide. The final
weighing is necessary because we need to subtract the original weight of magnesium
from this weight of product. We "weigh" the oxygen in this indirect fashion because it is
easier to do than to try to weigh the oxygen gas before it combines with the magnesium.
If magnesium is heated in open air, its reaction with oxygen is rapid and
spectacular. The metal catches fire, burning with an intense white flame and with
production of white smoke. (The metal has been used in flares to provide light for night-
time military operations.) We can slow down the reaction by limiting the supply of
oxygen that reaches the magnesium; we do that by putting a cover on the crucible that
contains the magnesium sample.
Our procedure in this experiment is complicated by another fact. Magnesium is
such an active metal that it reacts with the relatively inactive element nitrogen:
magnesium + nitrogen magnesium nitride (Rxn. 2)
This occurs in competition with the reaction of magnesium with oxygen, so it is called a
"side reaction." You should realize that this side reaction uses up some of the magnesium
that is supposed to be combining with oxygen. Fortunately it is possible to "undo" the
reaction in this way:
magnesium nitride + water magnesium hydroxide + ammonia (Rxn. 3)
and
magnesium hydroxide magnesium oxide + water vapor (Rxn. 4)
