Grade 11 Chemistry
Name: ____________________________
Determining the Percent Composition and the Empirical Formula
of Magnesium Oxide
Purpose: to calculate the percent composition and the empirical formula of an oxide of magnesium
Hypothesis:
1) Predict the chemical reaction that should occur when magnesium metal is reacted with oxygen gas.
2) Predict the theoretical percent composition by mass of the product you predicted in #1.
Materials: crucible and cover, clay triangle, iron ring, ring stand, stirring rod,
Bunsen burner, crucible tongs, dropper, ceramic pad, small beaker
Method:
1) Find the mass of a dry crucible and its cover, and record it in the Observations. Sand an 8 cm piece of
magnesium ribbon. Describe it, and then roll it into a coil, and place it in the crucible. Determine the mass
of the dry crucible, the cover and the magnesium and record it in the Observations.
2) Place the crucible on the clay triangle, supported on the iron ring on the ring stand. Put the cover on
slightly ajar, to allow air into the crucible. Heat strongly (top of the blue flame) for at least 8 minutes, or
until the magnesium is no longer smoking. Remove the cover and heat strongly for 5 more minutes.
3) Turn off the heat and allow the crucible and its contents to cool for 3 minutes on a ceramic pad.
4) When the crucible is cool enough to touch, carefully crush the contents with a stirring rod. Add 20 drops
of distilled water to wet all the surface of the powder, and to clean off the stirring rod.
5) Replace the crucible and its contents (with the cover slightly ajar) to the clay triangle, and heat the crucible
gently for 2 minutes to evaporate off the water. Then heat the crucible strongly again for 5 minutes (again
with the cover slightly ajar).
6) Allow the crucible and its contents to cool on the ceramic pad again, and then determine the mass of the
crucible, the cover and the magnesium oxide and record it in the Observations. Describe the product that is
produced.
Observations:
Make qualitative observations of the two reactants and the product. Also make quantitative observations
using the chart below:
mass of crucible and cover (g)
mass of crucible, cover and magnesium (g)
mass of crucible, cover and magnesium oxide (g)
Calculations:
Remember to carry at least four digits through each calculation, and to paraphrase all of your answers to the
correct number of significant digits.
1) Calculate the mass of magnesium you used, the mass of the magnesium oxide you produced, and the mass
of oxygen in the magnesium oxide you produced.
2) Calculate the experimental percentages of magnesium and of oxygen in your magnesium oxide. Show
your work for full marks.
3) Write your percent composition on the board along with the class. Copy the entire table into your report.
Calculate the average percent composition for the class. Show your work for full marks. Use this average
for your calculations in #4 and #5 below, and as the official result of the entire lab.
4) Calculate the empirical formula for the magnesium oxide you prepared in the lab. Remember to round to
the nearest whole number ratio, such as 1:1, 1:2, 2:2, 2:1, 3:3.
5) Calculate your percent error (PE) by finding the difference between the experimental mass percent (EP) of
magnesium and the theoretical mass percent (TP) of magnesium. Then you divide the difference by the
theoretical mass percent and multiply by 100 %.
PE = !EP – TP! x 100 %
TP
Analysis:
1) If you had accidentally lost some material due to spattering while heating, would your experimental
percentage of magnesium have been too high or too low? Explain, using numerical examples.
2) If all of the magnesium was not converted to magnesium oxide, how would this affect your experimental
percentage of magnesium? Explain, using numerical examples.
3) Is it possible that some of the magnesium melted and then boiled off during the heating process? Explain.
Conclusion:
Summarize your results, using the purpose as a guide.
Comment on your accuracy by referring to the % error calculated.
Comment on at least two errors that may have occurred during the lab.
Grade 11 Chemistry
Name: ______________________________
Determining the Percent Composition and the Empirical Formula
of Magnesium Oxide
Hypothesis
#1
#2
0 1 2 3
0 1 2 3
Observations
Qualitative
Quantitative
0 1 2
0 1 2 3
Calculations
#1
#2
#3
#4
#5
0
0
0
0
0
Analysis
#1
#2
#3
0 1 2 3
0 1 2 3
0 1 2
Conclusion
0 1 2 3 4
TOTAL :
Inquiry :
Communication:
1
1
1
1
1
2 3
2
2 3
2 3
2
_____ / 36
_____ / 10