Finding the Percentage of Oxygen in
the Atmosphere
Background Information
The
atmosphere of the earth is composed of a mixture of gases. The two most
abundant gases are nitrogen and oxygen. Oxygen also appears in the earth's
crust combined with other elements to form minerals.
In
this investigation, you will study how much of the air is consumed during
combustion. By measuring volume, you will be able to determine the percentage
of oxygen in the air.
Question
What
is the percentage of oxygen in the air?
Hypothesis-
see board
Test-
see board
Materials (per
group)
Tealite
candle
400-mL
plastic beaker
Flame
source- from instructor
Glass-marking
pencil
Large
Petri dish
Paper
towels
Water
Safety
goggles + aprons
Wire
or wood dowel Glass jar
Procedure- See diagram on board and set up from Demo
1. Using the formula for a
cylinder (eR2H)
measure the volume of cylinder using a ruler and metric units and record it in
the Data Table. The volume is also the volume of air in the jar at the start of
the experiment.
2. Pour
the water into the Petri dish. Carefully place the candle in the center of the dish.
Fill dish until the candle floats but leave space so as NOT to overflow the
dish when you place the jar over the candle.
3. Carefully
light the candle, and invert the jar over the lighted candle. Make sure that jar
rests on the piece of wire or wood so as not to form an airtight seal with
the bottom of the Petri dish. See Demo
4.
As the candle uses
the oxygen in the test tube, the candle will go out and water will be drawn
into the Jar to replace the oxygen.
5. When
the candle goes out, carefully mark the level of the water on the jar with the
glass-marking pencil. Remove the jar and find the height of the water, using
cm. Calculate the volume of Water( this
is also the volume of used up Oxygen) using (eR2H) with the new value of H and record in your
data table.
6. Dry
out your jar and dry off the candle and repeat the process for two more trials.
7. Determine
the percentage of oxygen in air by using the formula below. Record this
percentage in the Data Table.
% oxygen in air =
|
Volume
of oxygen in the test tube
|
X 100
|
Total
volume of air in test tube at start
|
Observations DATA
TABLE trial
1 trial 2 trial 3
Volume
of Air in the jar at Start
|
mL
|
|
|
Volume
of water in jar Tube After Candle Goes Out
|
mL
|
|
|
Volume
of Oxygen (should be same as above)
|
mL
|
|
|
Percentage
of Oxygen in Air
|
|
|
|
Conclusions
1. Would
the same result for the percentage of oxygen in air be obtained if a larger
test tube was used? A larger candle?
2. Why
does the water rise in the test tube as the candle goes out?
3. Nitrogen
is the other major component of air (78.1%). What property of nitrogen have you
discovered as a result of this experiment?
4. How
much oxygen is present in 5 L of air?
Critical Thinking and Application
1. Why
is oxygen such an important part of the earth's atmosphere?
2. Based
on your observations, what is an effective method of putting out a small fire?
3. "As
the altitude of an area increases, the density of the atmosphere
decreases." How can this statement be used to explain why it is more
difficult to breathe in Denver,
which has an altitude of more than 1500 meters, than in Houston, which is at sea level?