Determining the Empirical Formula of Magnesium Oxide

 

Objectives

 

Materials

 

 

Background

 

The gravimetric analysis involves the combustion of magnesium metal in air to synthesize magnesium oxide.  The mass of the product is greater than the mass of the magnesium used because oxygen bonds to the magnesium metal.  Like all gravimetric analyses, success depends on attaining a product yield near 100%.  Therefore, the product will be heated, cooled, and measured until two mass reading are within 0.02% of one another.  When the masses of the reactant and product have been carefully measured, then the amount of oxygen used 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.

 

Safety

1)      Always wear safety goggles and a lab apron to protect your eyes and clothing.

2)      If you get a chemical in your eyes, immediately flush the chemical out at the eyewash station while calling to your teacher.

3)      Know the location of the emergency lab shower and eyewash station and the procedure for using them.

4)      Do not touch or taste any chemicals.  If you get a chemical on your skin or clothing, wash the chemical off at the sink while calling to your teacher.

5)      Make sure you carefully read the labels and follow the precautions on all containers of chemicals that you use.  If no precautions are stated on the label, ask your teacher what precautions you should follow. 

6)      Do not taste any chemicals or items used in the laboratory. 

7)      Never return leftovers to their original containers; take only small amounts to avoid wasting supplies.

8)      When using a Bunsen burner, confine long hair and loose clothing.  Do not heat glassware that is broken, chipped, or cracked. 

9)      Use tongs or a hot mitt to handle heated glassware and other equipment; heated glassware does not always look hot.

10)  If your clothing catches fire, WALK to the emergency lab shower and use it to put out the fire.  Call to your teacher.  She may bring the fire blanket to you.

11)  Never put broker glass or ceramics in a regular waste container.  Broken glass and ceramics should be disposed of in the container designated for such broken materials.

12)  Dispose of used chemicals and products as directed by your teacher.

 

Procedure:

  1. Construct a setup for heating a crucible as showed in Figure A. and as demonstrated by your teacher.
  2. Strongly heat the crucible and lid for 5 minutes to burn off any impurities.
  3. Cook the crucible and lid to room temperature.  Measure their combined mass, and record the measurement on line 3 of the data table.  NOTE: Handle the crucible and lid with crucible tongs.  This prevents burns and the transfer of dirt and oil from your hands to the crucible and lid and therefore prevents additional experimental error.
  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, and place 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 the 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, as shown in Figure C.  CAUTION: Do not look directly at the burning magnesium metal.  The brightness of the light can blind your.
  7. When the magnesium appears to be fully reacted, partially remove the crucible lid and continue heating for 1 minute.
  8. Remove the burner from under the crucible.  After the crucible has cooled, use an eyedropper to carefully add a few drops of water, as shown in Figure D, to decompose any nitrides that may have formed.  CAUTION: Use care when adding water.  Too much water can cause the crucible to crack and your will have to start over.
  9. Cover the crucible completely.  Replace the burner under the crucible and continue heating for about 30-60 s.
  10. Turn off the burner.  Cool the crucible, lid, and contents 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 minutes.  Cool to room temperature and remeasure the mass of the crucible, lid, and contents.  Compare the mass measurement with the measurement of the mass in step 10.  If the new mass is +/- 0.02% of the mass in step 10, record the new mass on line 2 of the data table.  If not, your reaction is still incomplete.  Repeat this step.
  12. Clean the crucible, and repeat step 2-11 with a second strip of magnesium ribbon.  Record your measurements under Trial 2 in the data table.

 

Cleanup and Disposal

  1. Put the solid magnesium oxide in the designated waste container.  Return any unused magnesium ribbon to your teacher.  Clean your equipment and lab station.  Return shared equipment to its proper place.  Thoroughly was your hands after completing the lab session and cleanup.

 

Data Table

 

 

Trial 1

Trial 2

1. mass of crucible, lid, and metal (g)

 

 

2. mass of crucible, lid and product (g)

 

 

3. mass of crucible, and lid (g)

 

 

 

 

 

Analysis and Interpretation

(SHOW ALL WORK PROPERLY LABELED)

 

  1. Apply Ideas:  Calculate the mass of the magnesium metal and the mass of the product.
  2. Evaluating Data: Determine the mass of the oxygen consumed.
  3. Applying ideas:  Calculate the number of moles of magnesium and the number of moles of oxygen in the product.

 

Conclusions:

 

  1. Inferring Relationships:  Determine the empirical formula for magnesium oxide, MgxOy.  (Divide your mole ration by the moles of magnesium since this value is derived from a measure quantity instead of a calculated quantity.)
  2. Report your values to other lab groups using the overhead report sheet.  Include this data in your conclusion statement.