Determining the Empirical
Formula of Magnesium Oxide
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.
The following materials will be supplied for this lab:
Distilled water (in a small beaker)
Ceramic crucible with lid ($7.00 each, $14 both)
The following materials will be needed out of your equipment drawer:
Bunsen burner and sparker
Large ring, ring stand
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?
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.