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)
http://chemistry.about.com/od/chemistryfaqs/f/aircomposition.htm
At temperature level at 15°C and 101325 Pa (pressure):
1)
2)
3)
4)
5)
Nitrogen N2 = 78.084%
Oxygen O2 = 20.9476%
Argon Ar = 0.934%
Carbon Dioxide -- CO2 -- 0.0314% - carbon dioxide is a compound not an element therefore Neon is next…..
Neon Ne = 0.001818%
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 are on the left side of the equation:
2 Mg(s) + O2 (g); therefore magnesium ribbon and oxygen in the air.
The equations for the reactions are
2 Mg(s) + O2(g) --> 2 MgO(s )
3 Mg(s) + N2(g) --> Mg3N2(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)
“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.”
“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.”
4) Magnesium nitride is often formed in this lab, how and why does this occur? Include any
applicable chemical reactions. (2I)
Because Nitrogen is the most abundant diatomic molecule in the air at 78%, the magnesium ribbon will
react with the nitrogen as well as the oxygen in the air.
3 Mg(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)
Water is used with heat to get rid of the magnesium nitride. “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. 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.”
Magnesium nitride reacts with water to produce ammonia gas, as do many metal nitrides.
Mg3N2(s) + 6 H2O(l) → 3 Mg(OH)2 + 2 NH3 (gas)
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)
In this lab, magnesium metal (ribbon) is oxidized by oxygen gas to magnesium oxide (which is a compound).
Magnesium reacts vigorously when heated in the presence of air. The Mg-O2 reaction is energetic enough to
allow some Mg to react with gaseous N2. Although there is a higher percentage of N2 gas in the atmosphere than
O2, O2 is more reactive and the magnesium oxide forms in a greater amount than the nitride. The small amount
of nitride that forms can be removed with the addition of water, which converts the nitride to magnesium
hydroxide and ammonia gas. Heating the product again causes the loss of water and conversion of the
hydroxide to the oxide.
The unbalanced equations are:
Mg(s) + N2(g) + O2(g) ® MgO(s) +Mg3N2(s) (1)
MgO(s) +Mg3N2(s) + H2O(l) ® MgO(s) + Mg(OH)2(s) + NH3(g) (2)
MgO(s) + Mg(OH)2(s) ® MgxOy(s) + H2O(g) (3)
Based on the masses of the solid reactant and product, the mass in grams and the amount in moles of Mg and O
in the product can be determined:
mass of Mg + mass of O = mass of MgxOy - The empirical formula of magnesium oxide, MgxOy, can be written
based on the lowest whole-number ratio between the moles of Mg used and moles of O consumed.
7) Draw a flowchart for the procedure and create a data table for this experiment. (4 C)
Flow chart:
1) Determine mass of dry crucible and cover – record
2) Exact mass of crucible and magnesium ribbon
3) Set up on the retort stand
4) Heat strongly for 8 min. and adjust position if needed.
5) If magnesium still smokes replace cover. If not, continue heating for 5 min.
6) Turn burner off and cool for 3 min.
7) Stir contents to fine powder.
8) Add 20 drops water (distilled)
9) Heat gently for 2 min. to evaporate water.
10) Heat strongly for 6 min.
11) Cool to room temperature
12) Record mass of crucible, cover and Magnesium Oxide
Data Table:
Clean
Crucible
and lid
Clean Crucible and lid, Crucible and lid, with
with magnesium ribbon powdered magnesium
oxide
Crucible and lid and
contents after the
experiment: MgO(s)
Mass (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.
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)
2Mg(s)
+ O2(g)
-----------
Molar ratio: 2
Mass:
0.5 g
: 1
Molar mass:
24.31 g/mol
0.02056 mol
2
Number of
moles:
0.5g/24.31g/mol
0.01028 mol
moles = 0.02056 mol
X 16g/mol
= 0.1645408g
2MgO(s)
:2
2MgO(s)
Oxygen = mass(g) = number of moles X molar mass (g/mole)
0.02056 moles X 16 g/mole
= 0.32896 g
0.32896g/0.8287736g X100 = 39.69% for Oxygen
0.5g/0.8287736g X100 = 60% for Magnesium
2) Why must you take precautions to prevent the loss of a large amount of smoke? (2 A)
The magnesium oxide could have been lost through the crucible during the heating process. This
could result in the loss of the product. To prevent this, the lid should be set slightly off-centre on the
crucible so that only the air goes in. If too much smoke is lost, the product could be lost.
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)
We don’t have data from 4 other groups.**
Berthollet - devised a chemical nomenclature, or a system of names, which serves as the basis of the
modern system of naming chemical compounds.
Proust – “Law of definite proportions”; he showed that a given compound always contains the 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)
The percentage of magnesium would be too low because some of the product would have been lost.
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)
Then it would not be a 2:2 molar ratio. The number of particles would not be converted to MgO and
because the moles are less the mass would be affected and it too would be less.
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)
The boiling point of Magnesium is 1090 º C. This is very high and initially one would assume that the
temperatures of the Bunsen burner cannot go that level, however when checking resources it is stated
that.
“In a laboratory under normal gravity conditions and with a closed oxygen valve, a Bunsen burner burns
with yellow flame (also called a safety flame) at around 1,000 °C (1,800 °F).”
It is close to the temperature therefore it could be possible that some magnesium could be lost.
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
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