Measuring Percent Oxygen in Air

Chemistry 217

Dr.Rudolf Rednose

Tues/8am/S-473

Determining the Percent Oxygen in the Air

INTRODUCTION:

Recent tragedies in coal mines have exemplified the importance for us humans to breathe an adequate amount of oxygen if we are to survive. But what amount of oxygen is adequate? How can we know the amount of oxygen present in what we consider fresh air? The purpose of this experiment was to determine the percent oxygen typically present in air. The conclusion of the test suggested that the concentration of oxygen in the lab at the time of the experiment was around 20%.

METHOD:

The method for this experiment was to take advantage of oxygen's chemical reactivity with Iron. A piece of steel wool was placed in a test tube which was inverted in water. As the oxygen in the tube reacted with the steel wool the volume of air decreased in the tube until all the oxygen had reacted to form rust. The change in volume of air could then be determined to calculate the percent of O2 originally present.

RESULTS:

First Trial - Coarse Steel Wool

1. Measured the total height of the test tube; 14.5 cm.

2. Placed 0.55 g piece of coarse steel wool into solution of 1:1 H2O:Vinegar for aprox. 2 minutes.

3. Inserted steel wool into bottom of test tube.

4. Inverted test tube in 400 mL beaker that was filled to 250mL mark with H2O.

5. Vapor appeared around the steel wool as soon as it was placed in the test tube

6. Made observations as follows:

Coarse Steel Wool

Elapsed Time Decrease in Volume of Air (cm) % of 13.5 cm

5 minutes 0.5 cm 3.7

8 minutes 0.8 cm 5.9

11 minutes 1.0 cm 7.4

20 minutes 1.1 cm 8.1 % O2

7. Removed and dried steel wool.

8. Steel wool was brown, had a mass of 0.64 g

Second Trial - Fine Steel Wool

9. Measured the total height of the test tube; 14.5 cm

10. Placed 0.53 g piece of fine steel wool into solution of 1:1 H2O:Vinegar for aprox. 2 minutes.

11. Inserted steel wool into bottom of test tube.

12. Inverted test tube in 400 mL beaker that was filled to 250mL mark with H2O

13. Recorded Observations:

Fine Steel Wool

Elapsed Time Decrease in Volume of Air (cm) % of 13.5 cm

5 minutes 1.4 cm 10.4

8 minutes 1.7 cm 12.6

11 minutes 2.0 cm 14.8

14 minutes 2.2 cm 16.3

20 minutes 2.5 cm 18.5

25 minutes 2.6 cm 19.1% O2

Data Set

Trial Decrease in Volume of Air (cm) O2 % of Air

Coarse 1.1 cm 8.1 %*

Fine 2.6

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