Date: November 27, 2011
By: Swati Kulkarni
Lab Practice: Chemistry
PERCENT COMPOSITION OF MAGNESIUM OXIDE
To determine the percent composition by mass of a product and whether compounds have
a constant or variable composition
Background Knowledge:

Naming chemical compounds and their formulas

To know the types of chemical reactions

Stoichiometry of Chemical Reactions

Mole Ratios

Handling Chemical Equipments and know what are they called.
Safety Instructions:
 Do not look at the magnesium as it is burning because it generates white light
and it is mixed with UV light. It is unsafe to look at it directly.
 Behave responsibly in the lab all the times

Tie long hair back
 Wear Safety goggles and lab coat
 Know the location of the fire equipment; review use of that equipment before
beginning work.
 Read all the procedure thoroughly
 Be very careful while handling hot objects
 Do not place hot objects on the balance pan
 Be precise and accurate in measurements to obtain accurate results
Date: November 27, 2011
By: Swati Kulkarni
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)
The percent composition of the four most common elements in air is follows:
Nitrogen – 78.08 %, Oxygen – 20.95 %, Carbon dioxide – 0.033 %, Water vapour 1-3% (varies)
2) What are the reactants in this lab, what is the chemical reaction taking place (include correct
subscripts and balance)? (3 K/U)
The reactants in this lab are magnesium and oxygen.
The reaction taking place is: 2Mg (s) + O2 (g) → 2MgO (s)
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)
The cover of the crucible should be adjusted so that it is ajar to allow limited access of air.
4) Magnesium nitride is often formed in this lab, how and why does this occur? Include any
applicable chemical reactions. (2I)
Some of the magnesium reacts with nitrogen when we let air in the crucible.
3Mg (s) + N2 (g) → Mg3N2 (s)
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)
When we add 20 drops of distilled water and heat, magnesium nitride reacts with water to form
magnesium hydroxide and ammonia gas. When we heat the crucible strongly, ammonia gas is
driven off and at high temperature magnesium hydroxide decomposes to magnesium oxide and
water. We can get rid of water by heating and the only product remains is magnesium oxide which
is the desired product.
Mg3N2 (s) + 6 H2O (l ) → 3Mg(OH)2 (s) + 2NH3 (g)
Date: November 27, 2011
By: Swati Kulkarni
Mg(OH)2 (s) → MgO(s) + H2O (l )
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)
Based on the amount of reactants used (0.5g of magnesium reacts with oxygen) the only possible
formula for magnesium oxide is MgO. We can figure out how many moles of each of the reactants
are used as follows:
Calculate the mole of magnesium metal (mass/molar mass) = 0.485/24 =0.0202 mole
Calculate the mole of oxygen that reacted (mass/molar mass) = 0. 325/16 =0.0203 mole
which gives us approximately 1:1 mole ratio and therefore the only possible formula for
magnesium oxide is MgO.
7) Draw a flowchart for the procedure and create a data table for this experiment. (4 C)
Date: November 27, 2011
By: Swati Kulkarni
Data Table:
Mass of crucible and cover
32.345 g
Mass of crucible, cover and
magnesium ribbon
32. 830 g
Mass of magnesium ribbon
0.485 g
Mass of crucible, cover and
magnesium oxide
33.155 g
Mass of magnesium oxide
33.155 g - 32. 345 g = 0.810 g
Mass of oxygen that reacted
33.155 g - 32. 830 g = 0.325 g
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.
Date: November 27, 2011
By: Swati Kulkarni
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)
The percentage of magnesium in the compound can be found like this:
Percentage of magnesium in the compound =
=
mass of magnesium ribbon
mass of magnesium oxide compound
0.485 g
0.810 g
X 100
X 100
= 59.87 %
Percentage of magnesium in the compound = 59.87 %
Percentage of oxygen in the compound =
=
mass of oxygen
X 100
mass of magnesium oxide compound
0.325 g
0.810 g
X 100
= 40.12 %
Percentage of magnesium in the compound = 40.12 %
2) Why must you take precautions to prevent the loss of a large amount of smoke? (2 A)
The smoke consists of the magnesium oxide product and must not be lost from the crucible.
If a large amount of smoke is lost that will affect the accuracy of results and we will not have the
correct percent composition of magnesium and oxygen in Magnesium oxide.
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)
(As we do not have other four groups results, I have used fictional results)
Date: November 27, 2011
By: Swati Kulkarni
Observations ( our group # 1 )
Group # 2
Group # 3
Group # 4
Group # 5
Percentage of magnesium =
Percentage of
magnesium =
Percentage of
magnesium =
Percentage of
magnesium =
Percentage of
magnesium =
62.42%
68. 4 %
60. 17 %
65.56 %
Percentage of
oxygen =
Percentage of
oxygen =
Percentage of
oxygen =
Percentage of
oxygen =
37.58 %
31.6 %
39.87 %
35.45 %
59. 87 %
Percentage of oxygen =
40.13 %
From the above comparison table, Proust’s Law of Definite Proportions is evident. Proust showed that a
given compound always contains exactly the same proportion of elements by mass (The principle of the
constant composition of compounds)
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)
If we accidentally lost some material due to spattering in step 6, the percentage of magnesium
would be too high. This is because some of the magnesium oxide will be lost as light and we
would notice the percentage of magnesium to be quite larger.
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)
If all the magnesium is not converted to the oxide of magnesium, it would result in lower
percentage of magnesium in the oxide. As from 2 moles of magnesium, 2 moles of magnesium
oxide is formed it is in 2:2 molar ratio. If some of the magnesium does not convert to magnesium
oxide it will not be 2:2 molar ratio, and therefore it would be lower percentage of magnesium in
oxide.
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)
Magnesium has very high boiling point which is 1090 0c but as we heat strongly until all the
smoke is gone and then continue heating for 5 more minutes it could be possible to reach that
temperature and magnesium boils off during heating process.
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