Finding the Percentage of Oxygen in the Air
2014-15
Background Information:
The atmosphere of the earth is composed of a mixture of gases. The two most abundant are nitrogen and oxygen. Oxygen also appears
in the earth’s crust combined with other elements to form minerals. Oxygen is flammable and is required for a flame to be created.
Problem:
What percentage of the earth’s atmosphere is oxygen ?
Aim:
In the investigation, you will study how much of the air is actually consumed during combustion. By measuring volume, you will be
able to determine the percentage of oxygen in the air.
Materials:
candle
penny
beaker
marker
water
test tube
graduated cylinder
Procedure:
1. Light the candle and let a few drops of wax drop on the penny. Blow out the candle, and place the candle upright on the penny in the
wax. The candle should stick to the penny.
2. Measure 50 ml of water in the graduated cylinder. Pour the 50 ml of water into the beaker.
3. Carefully place the penny and the candle in the center of the beaker. The penny should keep the candle upright.
4. Light the candle carefully. Rapidly invert the test tube over the candle. Make sure the open end of the test tube is well under the
surface of the water, but not touching the bottom. See figure A.
5. As the candle uses the oxygen in the test tube, the candle will go out and the water will be drawn into the tube to replace the oxygen.
6. When the candle goes out, carefully mark the level of the water in the test tube with the marker. Remove the test tube.
7. Fill the test tube with water to this line. Pour this water into the graduated cylinder, measure the volume of water (ml) and record it
in the data table. This amount is the volume of air in the test tube that is not oxygen.
8. Empty the graduated cylinder. Fill the test tube completely with water. Pour this water into the graduated cylinder, measure the
volume of water (ml) and record it in the data table. This is the volume of air, including oxygen, in the test tube.
9. To find the volume of oxygen originally in the test tube, subtract the volume of air after the candle goes out from the total volume
that the test tube can hold. Record the volume in the data table.
10. 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
total volume of air in the test tube
x 100
Finding the Percentage of Oxygen in the Air
2014-15
Data Table
total
volume of air in the test tube after the candle goes out
ml
volume of air in the test tube
ml
volume of oxygen
ml
percentage of oxygen in the air (to the nearest tenth of a percent)
%
Conclusion: Answer the following in your own words.
1. Why does the water rise in the test tube as the candle goes out ?
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2. Look at the table on page 366 of your textbook. What 2 gases make up approximately 99% of the atmosphere ? What are the
percentages of each (do not round-off) ?
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3. What property of nitrogen have you discovered as a result of this experiment ?
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4. Would the same result for the percentage of oxygen in air be obtained if a larger test tube was used ? A larger candle ? Explain.
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5. What volume (ml) of oxygen is present in 500 ml of air ? Use the known percent oxygen rather than your data. _________________________
6. Calculate the percent error in your experiment. Use the following formula to calculate the percent error (use your value of % oxygen
and the actual value found in the book):
percent error =
% of oxygen in air (your value) – actual % of oxygen in air X 100
actual % of oxygen in air
percent error = ____________________