Name: ______

PERCENT COMPOSITION OF MAGNESIUM OXIDE

Introduction: The quantitative nature of chemistry was established by a number of chemists who lived between mid 1700s to mid 1800s. John Dalton revived the idea of the atom in the early 1800’s and he based much of his research on the work of his contemporaries. Two of his French contemporaries, J.L Proust and C.M Bertollet argued about the products of chemical reactions. Bertollet believed that a chemical compound could have an infinite number of possible combinations depending on the proportions of the elements that were used to prepare it. Proust believed that compounds had the same composition by mass regardless of the proportions of the elements that were used to prepare it. You will be preparing a compound and by comparing your results to those of your classmates, you will be able to determine whether Proust or Berthollet was correct.

You will be making a compound by reacting magnesium in air to make magnesium oxide. Some of the magnesium will form magnesium nitride which can be removed from the products by adding water and reheating the solid.

Purpose: To determine the percent composition by mass of a product and whether compounds have a constant or variable composition

Pre-Lab: /6 K/U/8 C (4 C for overall communication) /7 I

1)What is the percent composition of the four most common elements in air? (2 K/U)

2)What are the reactants in this lab, what is the chemical reaction taking place (include correct subscripts and balance)? (3 K/U)

3)Which direction is given in the lab to allow for the magnesium in the crucible to actually be able to react with the other reactant? (1 K/U)

4)Magnesium nitride is often formed in this lab, how and why does this occur? Include any applicable chemical reactions. (2I)

5)It is not ideal to have magnesium nitride in the final product, what chemical process is used to get rid of the magnesium nitride. Include any applicable chemical reactions (3I)

6)Do you think it is possible to have a variety of formulae for magnesium oxide based on the amount of reactants used? Why or why not? Clearly explain your answer. (2I)

7)Draw a flowchart for the procedure and create a data table for this experiment. (4 C)

Materials:

  • Crucible and cover
  • Clay triangle
  • Iron ring
  • Retort stand
  • Glass stirring rod
  • Bunsen burner
  • Crucible tongs
  • Dropper
  • Heating pad
  • Magnesium ribbon
  • Beaker
  • Water

Procedure:

1)Determine the mass of a dry crucible and cover separately. Record in your data table.

2)Place approximately 0.5 g of magnesium ribbon in the crucible. The exact mass of the crucible and magnesium must be recorded in your data table to the correct number of significant figures. Be sure to divide the magnesium ribbon into small pieces to increase the surface area of the reactant before recording the mass.

3)Place the covered crucible and contents on a clay triangle supported on a ring clamp on a retort stand. The cover of the crucible should be adjusted so that it is ajar to allow limited access of air. Heat strongly for about 8 min. If smoke escapes from the crucible, adjust the position (with crucible tongs!) to reduce the size of the opening.

4)After 8 min, remove the cover without adjusting the heat. If the magnesium starts to smoke, place the cover back on the crucible, leaving it slightly ajar. Continue to remove the cover periodically to check the progress of the reaction. If the magnesium still smokes, replace the cover, again slightly ajar. If there is not smoke when the cover is removed, continue heating the uncovered crucible strongly for 5 minutes.

5)Turn off the burner and allow the crucible and contents to cool for 3 min.

6)Use a glass stirring rod to carefully crush the contents of the crucible into a fine powder. Be aware that some of the product will stick to the stirring rod and try to minimize this effect. Add 20 drops of distilled water wet the surface of the powder. If any powder is present on the stirring rod, it can be washed off into the crucible using the 20 drops.

7)Replace the crucible cover slightly ajar and heat the crucible gently for 2 min to evaporate the water. Then heat the crucible strongly for 6 min. Be sure to burn off any soot deposits that might appear on the crucible.

8)Allow the crucible to cool to nearly room temperature. The cooling process will be faster if the crucible, cover, and contents are placed on the base of the ring stand. Determine the mass of the crucible, cover and oxide of magnesium

Analysis:

1)Calculate the percentages of magnesium and oxygen in the experimental oxide of magnesium. Show all calculations. (2 K/U)

2)Why must you take precautions to prevent the loss of a large amount of smoke? (2 A)

3)In a table, compare your percentages of magnesium and oxygen with those of 4 other groups. From the table, comment on the discussion about the composition of chemical compounds between Proust and Berthollet. (2 I, 2 A)

4)If you had accidentally lost some material due to spattering in step 6, would your percentage of magnesium have been too high or too low? Explain your answer clearly. (2 A)

5)If all of the magnesium was not converted to the oxide of magnesium in your experiment, how would this affect your calculated percentage of magnesium in the oxide? Explain your answer clearly. (2 A)

6)Would it be probable that some magnesium melted and then boiled off during the heating process in this experiment? Explain your answer clearly. (2 I)

Marking Scheme for Lab:

10 marks Communication based on communication rubric and proper lab format

2 marks Knowledge and Understanding in analysis

17 marks Application (2 data table, 8 analysis, 2 error analysis , 2 conclusion, 3 lab performance)

4 marks Inquiry in analysis

TOTAL /33