SCH4C     Lab #1   Determining the Percent Composition of a Compound

Purpose: The purpose of this lab is to first create an oxidized compound and then calculate the percentage of oxygen in it.

Safety Notes:  The burning of magnesium generates an intense white light.  Mixed in  with the white light there is ultraviolet (UV) light as well. Do not look at the magnesium as it is burning.

Apparatus: Retort stand, ceramic tile hot plate, ring clamp, pipe clay triangle, crucible and lid, crucible tongs, Bunsen burner.

Materials: 25 cm of Magnesium ribbon

1.   Obtain a 25 cm piece of magnesium ribbon. The exact length is not important.

2.   Find the mass of the magnesium ribbon and record it in the data table.

3.   Setup the apparatus with the retort stand, ring clamp, Bunsen burner and pipeclay triangle sitting on the ceramic tile hot plate. 

4.   Find the mass of the empty crucible and record it in the data table.

5.   Roll the ribbon of magnesium into a spiral or crush it into a compacted mass and place it in the crucible.

6.   Cover the crucible and heat strongly for 5 minutes or until red hot.

7.   With the crucible tongs gently lift the crucible lid to let air into the crucible.

8.   With the lid left slightly open discontinue heating.

9.   Remove the crucible lid and let the crucible cool before massing it again.

10.  Mass the crucible and contents. The contents are your new compound of magnesium oxide.  Record the mass on the data table.

Data Table:
Mass of magnesium ribbon           = __________
Mass of crucible                              = __________
Mass of crucible and contents       = __________

1.   Find the mass of the contents of the crucible. This is your new compound of magnesium oxide.

2.  The percentage of magnesium in the compound can be found like this:

     Percentage of magnesium =        mass of magnesium ribbon               X 100
                                                       mass of magnesium oxide compound

3.   Find the percentage of oxygen in the compound.

Conclusions: The percentage composition of magnesium oxide is ......

Error Analysis:  The above answers are your actual values.

If the formula for magnesium oxide is MgO use your periodic table to calculate the theoretical percentage composition of both the magnesium and the oxygen.

% error =        actual value - theoretical value        x 100
                                 theoretical value

Calculate the percentage composition for MgO.

Calculate the % error for O:

Calculate the % error for Mg: