A P CHEMISTRY
Determination of a Formula
Pre-Lab Questions - (Must be completed before lab work can begin.)
Calculate the percentage by mass of magnesium and oxygen in magnesium oxide, MgO.
Suppose 2.033 g of magnesium is heated in air. What is the theoretical amount of magnesium oxide that
should be produced?
Magnesium metal is a moderately reactive elementary substance. At room temperature,
magnesium reacts only very slowly with oxygen and can be kept for long periods of time without appreciable
oxide buildup. At elevated temperatures, however, magnesium will ignite in an excess of oxygen gas, burning
with an intensely white flame and producing magnesium oxide. Because of the brightness of its flame,
magnesium is used in flares and in photographic flashbulbs.
In this experiment, however, you will be heating magnesium in a closed container called a crucible,
exposing it only gradually to the oxygen of the air. Under these conditions, the magnesium will undergo a
more controlled oxidation, gradually turning from shiny metal to grayish-white powdered oxide. Because the
air also contains a great deal of nitrogen gas, a portion of the magnesium being heated may be converted to
magnesium nitride. Mg
, rather than magnesium oxide. Magnesium nitride will react with water and, on
careful heating, is converted into magnesium oxide
3MgO + 2NH
The ammonia produced by this reaction can be detected by its odor, which is released on heating the
Magnesium is a Group IIA metal, and its oxide should have the formula MgO. Based on this formula,
magnesium oxide should consist of approximately 60% magnesium by weight. By comparing the weight of
magnesium reacted, and the weight of magnesium oxide that results from the reaction, this will be confirmed.
Wear safety glasses at all times while in the laboratory.
Magnesium produces an intensely white flame if ignited, which may be damaging to the eyes. If the
magnesium used in this experiment ignites in the crucible by accident,
cover the crucible and
stop heating. Do not look directly at magnesium while it is burning.
When water is added to the crucible to convert magnesium nitride to magnesium oxide, the contents of the