THE
EMPIRICAL FORMULA OF MAGNESIUM OXIDE
Purpose
The purpose of this investigation is to determine the
empirical formula of magnesium oxide by burning a known mass of magnesium in air,
and using the known atomic masses of magnesium and oxygen to find the simplest
whole number molar ratio of the elemental constituents.
Instructions
Obtain a twenty-five centimeter sample of magnesium metal
(about 0.2 grams) and a crucible with lid.
Clean the crucible and lid by heating it on a clay triangle over a
Bunsen burner until the bottom of the crucible glows orange, then allow the
crucible to cool. Instructions will be
given on how to correctly handle the crucible with crucible tongs. Be sure not to touch the crucible or
lid with your fingers during the activity, which will affect the mass of the
crucible and its contents.
Find the mass of the magnesium to the closest 0.001
gram. Weigh the cooled crucible as
well. Cut up the magnesium strip into
small pieces and add them to the crucible.
Calculate the moles of magnesium by dividing the mass in grams by the
atomic weight of magnesium, 24.30 grams/mole, and report the moles (or millimoles, mmol) of magnesium.
Place the crucible on a clay triangle and heat it and its
contents with a Bunsen burner. The lid
should be on the crucible to prevent the loss of magnesium oxide ash, but it
should also be set to allow air to enter the crucible. Lift the lid slightly and briefly every five
minutes to allow more air in. If the
magnesium metal glows brighter when the lid is lifted then the reaction is not
complete. The reaction should take
around 20-30 minutes.
When all of the magnesium has reacted, cool the crucible
and lid and weigh them again. The added
mass compared to the empty crucible is the mass of the product. The difference between the mass of product
and the original mass of magnesium is the mass of oxygen added by the
reaction. Calculate the moles of oxygen
by dividing the mass by 16.00 grams/mole, and report the moles (or millimoles, mmol) of oxygen.
Find the ratio of magnesium to oxygen in the sample. Write the actual number of millimoles of magnesium to millimoles
of oxygen as mmol Mg : mmol O then divide both numbers by the smallest
value. The ratio will now be expressed
as either 1.00
: x.xx or x.xx : 1.00. Use this data to report an empirical formula
for magnesium oxide.
Preliminary Question
A crucible weighs 35.346 grams empty, 35.561 grams
containing magnesium metal, and 35.841 grams after a reaction with sulfur to
produce magnesium sulfide. What is the
empirical formula of magnesium sulfide?
Results
1. Mass of magnesium:
2. Moles of magnesium (mass of
magnesium/24.3):
3. Mass of crucible and lid:
4. Mass of crucible, lid, and product
(after heating):
5. Mass of product (Step 4 – step 3):
6. Mass of oxygen (mass of product – mass
of magnesium):
7. Moles of oxygen (mass of oxygen/16.0):
8.
Ratio
of moles of magnesium to Oxygen:
(Divide both numbers by the smallest number to get empirical
formula).
9. Formula of Magnesium hydroxide: