Determining the Empirical
Formula of Magnesium Oxide
Introduction
You
will be using Bunsen burners and ceramic crucibles to react magnesium with the
oxygen in our atmosphere. The magnesium
will also react with another atmospheric component; can you guess what that
might be? (Hint: it's almost 70% of our atmosphere) During the course of this lab you will be
working with very hot ceramics. It is
crucial to remember that ceramics remain dangerously hot long after they stop
glowing red! Pay close attention to
your instructor when proper safety precautions are demonstrated.
Materials
The
following materials will be supplied for this lab:
Magnesium
ribbon
Distilled
water (in a small beaker)
Disposable
pipettes
Safety
goggles
Electric
balances
Ceramic
crucible with lid ($7.00 each, $14 both)
Scissors
Litmus
paper
The
following materials will be needed out of your equipment drawer:
Bunsen
burner and sparker
Large
ring, ring stand
Ceramic
triangle
Crucible
Tongs
1) Check your crucible carefully for cracks or chips. If any are present, trade the crucible with
your instructor for an undamaged one.
Discoloration inside your crucible is harmless, and will not effect your
results.
2) Wash and dry the crucible and lid thoroughly. Place them in the ceramic triangle, above
the Bunsen burner, as illustrated here.
Tilt the lid to allow air to leave and enter the crucible freely. Heat the crucible to redness and allow to
cool. Keep the crucible in the triangle
during all cooling steps.
3) While the crucible is cooling, obtain a segment of magnesium
ribbon from your instructor. Cut it up
into small pieces, about as long as your little finger is wide. Do this over a sheet of clean binder paper
so the pieces can be easily swept into the crucible after it cools.
4) Take your crucible, lid, and your magnesium (still on the
binder paper) to a balance. Be sure the balance has been zeroed before every
mass determination! Weigh the crucible
and lid and record the mass. Add the Mg
to the crucible and record the new mass.
5) Heat the crucible with the magnesium at low heat for one minute, then medium heat for three minutes. Be sure the lid is tilted to allow free entry of air. Heat for an additional five minutes at high heat. Reduce heat gradually over a two minute period before shutting the bunsen burner all the way off. Allow the crucible to cool ten minutes. The contents should be a gray-black ash.
6) Add just enough water to moisten the gray ash in the
crucible. There may be a little
standing water in the crucible, but at least half of the ash should be above
the level of the water.
7) Heat the crucible very gently.
It may be useful to hold the Bunsen burner by the base and waft the
flame under the crucible. Try not to
lose any product due to spattering.
Carefully hold a piece of litmus paper in the steam as the water
evaporates. What color does it turn?
8) When all water has been evaporated, heat at medium for one
minute, then strongly for five more minutes.
Allow to cool for ten minutes.
9) Get the final mass of the magnesium ash and the crucible. Did it get lighter or heavier?
Data Analysis
a) Determine the mass of the magnesium ribbon. How many moles of magnesium did you start
with?
b) Determine the mass of oxygen that reacted with the
magnesium. How many moles of oxygen
were added to the magnesium?
c) Calculate the ratio of magnesium moles to oxygen moles.
d) Use the ratio from (c) to write the formula for Magnesium
Oxide. Does your formula agree with the
formula predicted by the ion charges of magnesium and oxygen?
Results and Conclusions
In your conclusion, you must
address the following:
a) What are the major sources of error in this experiment? (HINT: A major byproduct of this reaction is
between magnesium and another component of our atmosphere)
b) What safety cautions would you emphasize for those who next
perform this lab?
c) What was the purpose of the water addition? (HINT: Ammonia, NH3, is basic)
(Steps 6-8)
d) Finally, compare your experimental formula for magnesium oxide
with the predicted formula.