Name_____________________________ Period____
Parner(s)______________________________________
Lab 8-5: Empirical Formula of Magnesium Oxide
Background
Gravimetric analytical methods are based on accurate and precise mass measurements. They are used
to determine the amount of a compound or element in a sample material. A gravimetric procedure
involves reacting a sample of known mass to produce a product of known mass. The known mass can be
then used to calculate the mass of another element or compound in the sample. This gravimetric
analysis involves the synthesis (chemically combining elements to form a compound) of magnesium
oxide. Metal oxides are formed when a metal reacts with oxygen. In this lab, a known amount of
magnesium reacts with oxygen in the air to produce magnesium oxide. By measuring the mass of the
product, the amount of oxygen reacted with magnesium can be determined and then the formula of
magnesium oxide can be determined empirically (experimentally).
Safety
Observe all lab precautions, especially the ones listed below.
 Wear your safety goggles at all times.
 Tie back loose hair and confine loose clothing.
 Hot and cold crucibles look the same – use tongs when handling the crucible and lid.
 Wash hands thoroughly before leaving the lab.
Materials
Safety goggles
Crucible
Crucible lid
Clay triangle
Ring stand
Iron ring
Bunsen burner
Balance
Figure 1.
Magnesium ribbon
Crucible tongs
Steel wool
Wire gauze
Procedure
___1. Clean a crucible with soap and water. Once the crucible is clean, do
not touch the crucible with your fingers for the rest of the
experiment because the salts from your skin can change the mass of
the crucible.
___2. Place the crucible and lid on the clay triangle as shown in figure 1.
___3. Position the lid slightly tipped, leaving only a small opening for gases to escape
as shown in figure 2.
___4. Heat the crucible and lid for 3-5 minutes.
___5. Remove from heat and allow to cool for 3-5 minutes.
___6. While the crucible and lid are cooling, obtain approximately 10cm of
magnesium ribbon. Using steel wool, polish the magnesium ribbon. Tear
the magnesium into two pieces. Gently wrap the magnesium around your
pinky so that it is coiled.
___6. When the crucible is cool, obtain the mass of the crucible and lid.
Record this mass in your data table.
Figure 2.
___7. Place the magnesium ribbon into the crucible. Determine the mass of the crucible, lid, and
magnesium. Record this mass in your data table.
___8. Place the crucible on the clay triangle. Heat the lid and crucible, lifting the lid occasionally as
shown in figure 2.
CAUTION: DO NOT LOOK DIRECTLY AT THE BURNING MAGNESIUM!
Be careful to not lose the smoke – this means you are losing magnesium!
___9. When the magnesium appears to be fully reacted (looks like ashes and no longer smoking),
remove the crucible lid (set it on the wire gauze) and continue heating for 5 minutes.
___10. Remove from heat and allow all contents to cool. Once the crucible is cool, obtain the mass of
the crucible, lid and product. Record this mass in your data table.
___11. After mass is obtained, place the magnesium oxide in the marked disposal container. Wipe
down your lab station and return the crucible and lid to the bin.
___12. Wash your hands after cleaning up your lab station.
Data and Observations (5 points)
Observations:
Lab Data
Remember units and sig figs!!
Mass of crucible and lid
Mass of crucible, lid, and magnesium
Mass of crucible, lid, and magnesium oxide
Calculations (11 points)
SHOW ALL YOUR WORK!
Calculated Data
Be sure to show the actual values that you need. Remember units and sig figs!!
Mass of magnesium
Mass of oxygen
Calculation of Empirical Formula based on YOUR LAB DATA
My empirical formula (final answer) is
Analysis and Conclusion
1. (1 point) What is the accepted formula for magnesium oxide? Hint: Use the periodic table!
2. (1 point) Did you have too little or too much oxygen?
3. (2 points) Identify one experimental source of error for this experiment. Explain the impact
of the error on your results. (Be specific! Do not list calculation and/or misreading the scale!
These are not experimental!)
4. (4 points) Write the balanced reaction for this experiment. Be sure to include ALL symbols!!
5. (1 point) Why did we use a lid in this experiment but not in the decomposition of KClO3 lab?
6. (1 point) Why did we have to occasionally lift the lid during the experiment?
7. (8 points) Due to time restraints for our class, we had to modify/change some of the
procedures. Read the following paragraph then answer the questions that follow.
When magnesium burns in the air, it also reacts with nitrogen, forming magnesium nitride.
Because this side reaction imposes error in the empirical formula calculations, the magnesium
nitride needs to be eliminated without losing any magnesium. Nitrides react with water,
producing aqueous nitrogen trihydride and solid magnesium hydroxide. Further heating drives
off the ammonia as a gas and decomposed the magnesium hydroxide. The water formed as a
product of the decomposition turns into a gas during heating. This leaves magnesium oxide, the
final product in the crucible.
Write the balanced reaction for each side reaction.
a. Magnesium reacts with nitrogen
b. Decomposition of magnesium hydroxide (use your reaction code sheet!)
* THIS MUST BE COMPLETE
ON TIME IN ORDER TO
Name______________________________ Period____
PARTICIPATE IN THE LAB!!
Partner(s)______________________________________
Pre Lab 8-5: Empirical Formula of Magnesium Oxide
You will likely need to reference back to empirical formula (ch7,pt2) notes to complete this pre lab.
1. (1 point) What is an empirical formula?
2. (11 points) Practice your calculations! Iron is heated in the presence of oxygen to form iron
oxide. Find the empirical formula of iron oxide based on the lab data below.
Mass of crucible and lid
36.170 grams
Mass of crucible, lid, and iron
38.200 grams
Mass of crucible, lid, and iron oxide
38.970 grams
Calculated Data
Mass of iron
SHOW ALL WORK!
Remember units and sig figs!!
Mass of oxygen
Calculation of Empirical Formula based on YOUR LAB DATA
My empirical formula (final answer) is
3. (2 points) Why is the final mass larger than the mass before it in the data table?
4. (2 points) What are two possible formulas that you could between iron and oxygen (based on
the charges of iron)?
5. (2 points) Assume that your empirical formula should have been Fe2O3.
Did you have too little or too much oxygen? Explain.
6. (2 points) Give an explanation of why the results did not work out perfectly (What is the most
likely source of experimental error for burning of iron in oxygen? Explain the impact on your
results.)