Monday, February 4, 2013

Earth science - % oxygen minilab

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.


      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
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
Volume of water in jar Tube After Candle Goes Out
Volume of Oxygen (should be same as above)
Percentage of Oxygen in Air



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?


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