Determination of Percent Composition of MgO–Law of Definite Proportions

Lab: Law of definite proportions

Percent composition of Magnesium oxide

In this lab you will test and validate the law of definite proportions. To do this you will determine the composition by mass of magnesium oxide, calculate the percent composition and compare your results with other groups working on the same experiment.

Problem:

What is the percent composition by mass of Magnesium oxide? Is the percent composition a constant? Is the value comparable with the literature value?

Prediction / Hypothesis:

Make a prediction based on the law of definite proportions. What should the percent composition by mass of Magnesium oxide be?

Experimental Design:

A know mass of magnesium metal is heated in a crucible over a laboratory burner. The mass of the magnesium oxide formed is used to determine the mass of the oxygen that reacted with magnesium. Use this information to determine the percent composition by mass of magnesium and oxygen in magnesium oxide formed. Some of the magnesium also reacts with nitrogen in air to form a nitride. This compound is converted to magnesium oxide by adding water and reheating the solid.

Materials:

  • Eye protection Goggles
  • Chemical or electronic balance
  • 1 to 2 cm magnesium ribbon
  • Sand paper to polish the magnesium strip
  • Porcelain crucible and lid
  • Laboratory burner / Bunsen burner
  • Retort stand or tripod
  • Ring stand and clamp
  • Clay triangle
  • Glass stirring rod
  • Distilled water

Procedure:

Step A

  • Put on safety goggles once you start the lab and take it off only when all your work is done.
  • Using a balance, determine the mass of a clean, dry porcelain crucible and lid.
  • Polish the magnesium ribbon with the steel wool and fold the ribbon to fit into the bottom of the crucible.
  • Determine the mass of the magnesium strip using the electronic balance. Use 4 decimal places for the mass.
  • Attach the ring clamp to the retort stand about 10 cm above the top of the Bunsen burner.
  • Place the crucible securely on the clay triangle resting on the ring clamp. Place the lid on the crucible so that it is slightly ajar. This is necessary to allow oxygen to enter the crucible.
  • Practice covering and uncovering the crucible using tongs.
  • Light the burner and heat the magnesium gently using a clean blue flame. You may have to raise the lid periodically using tongs, to see if the reaction is proceeding.
  • After about 2 minutes of gentle heating, increase the intensity of the flame. Heat strongly until the magnesium no longer flares.
  • Turn off the burner and allow the crucible and its contents to cool.
  • Determine and record the mass of the contents of the crucible.
  • Save the contents of the crucible for part B

Step B

  • Using tongs, transfer the cooled crucible to the table top. Crush the white ash in the crucible with the end of a stirring rod.
  • Add about 3 mL of distilled water to the crucible. Use some of the water to wash any residue on the rod into the crucible.
  • Heat the crucible and its contents gently for about 2 minutes to evaporate the water. Then heat strongly for 6 minutes to convert any magnesium nitride present to magnesium oxide.
  • Allow the crucible and its contents to cool.
  • Determine and record the mass of the contents of the crucible.
  • Discard the contents of the crucible as directed by your teacher.

Analyze and Evaluate

  • Calculate the theoretical percentage of composition of magnesium oxide MgO
  • Determine the mass of Oxygen that reacted with magnesium in Step A and Step B.
  • Calculate the percentage composition of MgO from the data obtained from the experiment in Step A and Step B
  • Compare the three different values for percentage composition of magnesium oxide. Explain the differences you may have observed.
  • If there were any differences between theoretical value and experimental values, explain the discrepancy in values, suggest sources of error that may have contributed to the differences in the values obtained.
    • Separate the sources of error into
      • Avoidable errors and
      • Unavoidable errors
  • Suggest modifications to the experiment, so that your results may be more accurate.


Submit the following date sheet to your teacher at the end of the lab

Item #

Data Being Collected

Mass in grams

(use 4 decimals)

1

Mass of empty dry crucible with lid

 

2

Mass of Magnesium metal

 

3

Mass of crucible lid and magnesium oxide in Part A of the experiment

 

4

Mass of crucible lid and magnesium oxide in Part B of the experiment

 

5

Mass of oxygen reacted with magnesium in

Part A

 

6

Mass of oxygen reacted with magnesium in

Part B

 

7

Theoretical mass of oxygen that reacted with magnesium.

 

Write a Formal Lab Report using the template provided