Jan 26 2014
Determining the Empirical Formula of Magnesium Oxide
Background: This gravimetric analysis involves the combustion of magnesium metal n air to synthesize magnesium oxide. The mass of the product is greater than the mass of magnesium used because oxygen reacts with the magnesium metal. As in all gravimetric analyses success depends on attaining a product yield near 100%. Therefore, the product will be heated and cooled and have its mass measured until two of these mass measurements are within 0.02% of one another. When the masses of the reactant and product have been carefully measured, the amount of oxygen use in the reaction can be calculated. The ratio of oxygen to magnesium can then be established, and the empirical formula of magnesium oxide can be determined.
Aim: Determining the empirical formula of magnesium oxide.
Materials Required: 1. 15 cm magnesium ribbon, 2
2. 25 mL beaker
3. Bunsen burner assembly
4. Clay triangle
5. Crucible and lid, metal or ceramic
6. Crucible tongs
7. Distilled water
8. Eyedropper or micropipet
9. Ring stand
Procedures: 1. Construct a setup for heating a crucible as shown in Figure A and as demonstrated in the Pre-Laboratory Procedure “Gravimetric Analysis.”
2. Heat the crucible and lid for 5 min to burn off any impurities.
3. Cool the crucible and lid to room temperature. Measure their combined mass, and record the measurement on line 3 of your data table.
NOTE: Handle the crucible and lid with crucible tongs at all the time during this experiment. Such handling prevents burns and the transfer of dirt and oil from your hands to the crucible and lid.
4. Polish a 15 cm strip of magnesium with steel wool. The magnesium should be shiny. Cut the strip into small pieces to make the reaction proceed faster the pieces in the crucible.
5. Cover the crucible with the lid, and measure the mass of the crucible, lid, and metal. Record the measurement on line 1 of your data table.
6. Use tongs to replace the crucible on the clay triangle. Heat the covered crucible gently. Lift the lid occasionally to allow air in. CAUTION: Do not look directly at the burning magnesium metal. The brightness of the light can blind you.
7. When the magnesium appears to be fully reacted, partially remove the crucible lid and continue heating for 1 min.
8. Remove the burner from under the crucible. After the crucible has cooled, use an eyedropper to carefully add a few drops of water to decompose any nitrides that may have formed . CAUTION: Use care when adding water, Using too much water can cause to crucible to crack.
9. Cover the crucible completely. Replace the burner under the crucible, and continue heating for about 30 to 60s.
10.Turn off the butter. Cool the crucible, lid, and contends to room temperature. Measure the mass of the crucible, lid and product. Record the measurement in the margin of your data table.
11. Replace the crucible, lid, and contents on the clay triangle, and reheat for another 2 min. Cool to room temperature, and remeasure the mass of the crucible, lid and contents. Compare this mass measurement with the measurement obtained in step 10. If the new mass is +/_0.02% of the mass in step 10, record the new mass on line 2 of your data table and go on to step 12. If not, your reaction is still incomplete, and you should…