Determination of Empirical Formula


·  Compute the empirical formula of magnesium oxide

·  Calculate percent error

·  Explain reason for percent error


In this lab you will experimentally determine the empirical formula of magnesium oxide, the compound formed when magnesium metal reacts with oxygen.


25cm ribbon of Mg


Safety goggles 1 ring stand

1 crucible 1 ring support

1 crucible lid triple beam balance

1 crucible tongs 1 bunsen burner

1 clay triangle


Wear safety apparel

Do not look directly at burning magnesium

Do not inhale the smoke produced when the magnesium is burned

Handle crucible and lid with crucible tongs only—may reach temperatures above 3000C


As you perform the experiment, record your data in a table similar to the one shown below

1.  Set up equipment as demonstrated by the instructor

2.  Clean a crucible and lid with water. Dry them by heating n the hottest part of the flame for 5 minutes. Remove from heat and allow them to cool for at least 10 minutes.

3.  Measure the combined mass of the crucible and the lid to two decimal places. Record.

4.  Place a coiled 25-cm length of magnesium ribbon in the crucible. Measure and record the combined mass of the crucible, lid, and magnesium.

5.  Lab partner #1 will set the uncovered crucible (containing magnesium) on the clay triangle. Start a high flame. When the Magnesium ignites, turn off flame.

6.  Lab partner #2 is poised holding the crucible lid with a crucible tongs. When the magnesium ignites, immediately cover crucible with lid slightly askew.

7.  When smoke production has cease, replace the burner and continue heating. The object is to convert all magnesium ribbon into a light gray powder, magnesium oxide. This should take around 10 minutes to accomplish. Check every few minutes, once you observe no ribbon—the reaction is complete.

8.  Turn off and let cool for at least 10 minutes.

9.  Measure and record the combined mass of the crucible, crucible lid, and magnesium oxide.

10.  Follow your teacher’s instructions for proper disposal of the materials.

Data Record:

Data table
Item Mass
Empty crucible and lid
Crucible, lid, and Mg (before heating)
Crucible, lid, and combustion product (MgxOx)

Data Analysis:

1. Determine the mass of magnesium used

2. Determine the number of moles of magnesium used. Hint: mol Mg = (mass Mg/molar mass Mg).

3. Determine the mass of magnesium oxide formed.

4. Determine the mass of oxygen that combined with magnesium.

5. Calculate the number of moles of oxygen atoms that were used.

6. Calculate the ration between moles of magnesium used and moles of oxygen used. Express this ratio in simplest whole-number form.

7. Based on your experimental data, write the empirical formula for magnesium oxide.

8. Calculate the percent error in your determination of the magnesium:oxygen mole ration, using the accepted value provided by your teacher.

Percent error = ½accepted value-experimental value½ X 100 percent

Accepted value

9. Identify major sources of error in this experiment. Explain how the magnesium:oxygen ratio would be affected by each error you identify.

Additional Empirical Formula Problems: