Experiment 2.2
Law of Definite Composition (4)
Report=98
Introduction: (5)
Elements are a kind of matter that cannot be
decomposed by ordinary chemical means. Compounds
are chemical combinations of elements. What is a
chemical combination as opposed to just any
combination of elements? It would help to give an
example of what is not a compound. The combination of
oxygen and nitrogen in the air is a mixture of the gases.
This mixture can be in all proportions of oxygen and
nitrogen. The properties of the mixture depend on the
properties of the individual components of the mixture.
Compounds, on the other hand, are special combinations
of the elements. A compound has its own properties,
distinct from the properties of its elements.
The law of definite composition states that the elements
forming a compound always combine in the same
proportion by mass. The compound water, H2O, is
always a chemical combination of hydrogen and oxygen
in a 1:8 ratio by mass. If a mixture of hydrogen and
oxygen were reacted, in some other mass ratio, for
example 1:2, water would be formed but some hydrogen
would remain unreacted. Water forms only in the 1:8
ratio by mass.
In this experiment, you will examine the reaction between
magnesium metal, Mg, and oxygen O2. When heated,
magnesium reacts readily with oxygen in the air. The
magnesium will be heated strongly in an open crucible
for several minutes. You will measure the mass of the
magnesium that reacts and the mass of magnesium
oxide that is formed, and use your data to calculate the
mass of the oxygen that combined. You can then find
the ratio of mass of magnesium to mass of oxygen and
compare your experimental ratio with the ratios of your
classmates and with the actual ratio. By calculating the
percent error you can then evaluate how accurately you
performed the experiment and look for sources of error
Objectives: (2)
1. Observe a reaction between magnesium
and oxygen.
3. Measure masses carefully to obtain
accurate results..
2. Calculate a ratio of mass of magnesium to
mass of oxygen..
Materials: (5)
Apparatus:
laboratory apron
sandpaper or emery cloth
wire gauze
Bunsen burner and lighter
ring stand and ring
pipestem triangle
analytical balance
medicine dropper
crucible tongs
crucible and lid
safety glasses
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Reagents:
magnesium ribbon
deionized water
Procedure:
1. Adorn your safety glasses and lab apron.
2. Obtain a piece of magnesium ribbon between 10 cm
and 15 cm long from your teacher. If the surface of the
magnesium is not shiny, use a piece of steel wool to
shine the surface. Set up the ring stand, ring, burner,
and clay triangle as show in figure A
3. Obtain a clean, dry crucible and cover, then heat over
burner for 4 min. Let cool. Find the mass of the
Experiment 2.2
Rev
crucible and cover to the nearest 0.0001g and record it
on the Report Sheet.
4. Fold the magnesium in an accordion shape, to
provide maximum surface area, and place it into the
crucible. It is ok if the magnesium brakes, just add it
all to the crucible. Find the mass of the crucible, cover,
and magnesium. Record it on the Report Sheet.
5. Place the crucible on the clay triangle. Begin heating
the crucible gradually with the lid completely on. Heat
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slowly by moving the flame around underneath the
crucible. Remove the heat temporarily if a large
amount of smoke comes out of the crucible.
7. Turn off the burner and put the lid back on the
crucible. Place crucible and cover onto ceramic title.
Allow the crucible and cover to cool to a temperature
low enough so that you can touch the crucible. Find
the mass of the crucible, contents, and cover. Record
the mass on the Report Sheet.
8. Add ten drops of deionized water. Smell cautiously,
noting any odor. Set up the crucible for heating again.
Reheat for four more minutes with the lid on. Let the
crucible cool once again.
9. Find the mass of the crucible, cover, and product.
Record it on the Report Sheet.
Figure A
6. After about 4 min. of direct heating with no smoke,
remove the lid slightly. Heat the crucible to redness for
4 min. Finally remove the lid completely and heat
strongly for four more minutes.
10. Compare the masses found in steps 7 and 9. If the
masses do not agree within 0.005 g, reheat the
crucible for 4 min, cool, and find the mass. Repeat
until the last two masses agree within this range.
11. Before leaving the laboratory, clean up all materials
and wash your hands thoroughly.
Data and Observations: (28)
(8) Data Table 1 - Primary Data
Mass of crucible and cover
Mass of crucible, cover, and magnesium
Mass of crucible, cover, and product after first heating
Mass of crucible, cover, and product after second heating
Mass of crucible, cover, and product after third heating
(20) Stamp ...........................
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Calculations (12) - Show all derivations, then record results in Table 2:
(2) 1. Find the mass of magnesium that reacted?
(2) 2. Find the mass of the magnesium oxide that
was produced.
(1) 5. According to the periodic table, what is the
mass of a magnesium atom?
(1) 6. According to the periodic table, what is the
mass of an oxygen atom?
(2) 3. Find the mass of oxygen that reacted.
(2) 4. Find the ratio of the mass of magnesium to
the mass of oxygen that reacted to form
magnesium oxide, i.e. the experimental value.
Experiment 2.2
Rev
(2) 7. Calculate your percent error using the formula
|Exp Value - Acc Value| * 100 = %error
Acc Value
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(5) Data Table 2 - Secondary Data
Mass of magnesium oxide produced
Mass of magnesium that reacted
Mass of oxygen that reacted
Exp ratio of mass of magnesium to mass of oxygen reacted
% error
Analysis and Conclusions: (17)
(4) 1. Check with other members of your class to
see how your Magnesium/oxygen ratios
compare. What is the range of values. Do class
results verify the law of definite composition.
(4) 2. How would your magnesium/oxygen ratio be
affected if not all of the magnesium reacted?
Class Data Mg/O Ratio (9)
Group
Ratio
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:
Group
Ratio
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:
Group
Ratio
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:
Group
Ratio
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:
:
Mean = ___________
Range: ____________
Synthesis: (10)
(2) 1. What is the accepted formula for magnesium
oxide?
(2) 2. Use the accepted value of the Mg/O mass
ratio to determine what mass of magnesium
would react with 16.0 grams of oxygen. Show
derivation.
3. Suppose you tried to react a combination of 42.0
grams of magnesium and 45.0 grams of
oxygen.
(2) a. Of the two substances, which would not be
completely consumed in the reaction?
Explain.
Experiment 2.2
(2) b. How much magnesium oxide would be
formed?
(2) 4. When one pound of gasoline, a mixture of
hydrocarbons, is burned in an automobile,
approximately 3 pounds of carbon dioxide,
CO2, is given off. Carbon dioxide is one of
the gases contributing to global warming.
What information from this experiment helps
to explain how one pound of gasoline can
give off approximately 3 times as much CO2?
Hint: look at periodic table for each element.
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