Name:
Date:
CHAPTER 26: FINDING THE PERCENTAGE OF OXYGEN IN THE ATMOSPHERE
Background Information
The atmosphere of the Earth is composed of a mixture of gases. Gas types and amounts are
dependent on which layer of the atmosphere you are referencing. The two most abundant gases in
the troposphere are nitrogen (N2) and oxygen (O2). Oxygen also appears in the Earth's crust. Here it is
combined with other elements to form minerals. In this particular lab investigation, you will study
which specific troposphere gas is consumed during combustion (burning) and how much of it is lost
during this process. By measuring different volumes, you will be able to determine the percentage of
oxygen present in the troposphere.
Materials (per group)
Birthday Candle
Small beaker—100mL
100-mL graduated cylinder
Matches
Glass-marking pencil or marker
Large test tube
Penny
Water
Clay
Directions
1. Set up your candle apparatus. Place a small ball of clay on the penny. Insert your candle into the
clay so it is stable and perpendicular to the base of the penny.
2. Fill the entire test tube with water--all the way to the top. Pour this water into the graduated
cylinder. Place the graduated cylinder on a level surface and measure the volume of water and then
record this value in mL into the data table--line 1. This volume represents the volume of air in the test
tube at the start.
3. Slowly pour this water into the small beaker. Carefully place the candle apparatus in the center of
the beaker. The penny and clay should keep the candle upright.
4. Carefully light the candle, and rapidly invert the test tube over the lighted candle. Make sure that
the open end of the test tube is well under the surface of the water but not touching the bottom
of the beaker. This has to be done very quickly and carefully to get the desired results!!!
5. As the candle uses up the oxygen present in the test tube (combustion), the candle will burn out
and water will be drawn up into the tube to replace the oxygen burned. When the candle goes out,
carefully mark the level of the water in the test tube with the glass-marking pencil or marker. Then,
remove the test tube.
6. Fill the test tube up with water only up to the marked line. Using the graduated cylinder on a level
surface, measure this volume of water and record it into the data table--line 2. This volume is the
volume of air left in the test tube after the candle used up all the oxygen during combustion.
Therefore, line 2 is the volume of nitrogen and all the other gases present in the troposphere. Note:
the volume in line 2 MUST be less than the volume in line 1.
7. To find the volume of oxygen originally in the test tube, subtract line 2 from line 1, and enter the
difference into the data table--line 3. Line 3 represents the volume of oxygen used during
combustion.
8. Determine the percentage of oxygen in air by using the formula below. Record this percentage in
the data table--line 4.
% oxygen in air
=
volume of oxygen in test tube
total volume of air in test tube at start
x 100
Observations
Line 1: Volume of Air in Test Tube at Start
Line 2: Volume of Air in Test Tube After Candle Goes Out
Line 3: Volume of Oxygen
Line 4: Percentage of Oxygen in Air
DATA TABLE
mL
mL
mL
%
Conclusions
1. Write down the calculated percentages of oxygen derived by each of the lab
groups in your section. Calculate the average percentage based on your class'
results.
Other groups' results:
Class Average:
%
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%). What property of nitrogen
have you discovered as a result of this experiment?
4. How much oxygen is present in 5 liters (L) of air? Show all of your work.
5. 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 at Cape May, which is
at sea level?