Title: Determination of percentage oxygen in air
Aims: To determine the percentage of oxygen in air
To observe an oxidation reaction
Target Group: Transition Year
Equipment (per group): test tube, forceps, water, water trough or large
beaker, fine grade steel wool, ruler
Chemicals (per group): acetone (Approx 30mls), acetic acid (Approx 30mls),
Background
Iron reacts with oxygen to form iron oxide. In the reaction the oxygen will be
consumed. Volume of oxygen should decrease as the reaction proceeds.
Water should therefore enter the test tube. The change in volume of water will
be equal to the volume of oxygen consumed in the reaction. The volume of
oxygen in the air can be determined from this information
Safety Analysis
Lab coat and safety goggles should be worn at all times. Take care when
handling acetone as its fumes are harmful (causes irritation to eyes skin and
respiratory tract). Rinse steel wool in fume cupboard and ensure excess
acetone is shaken off. Acetone is also highly flammable so should not be
used near any naked flames. All spillages should be reported to the teacher
immediately. Gloves should be worn at all times. Take care when handling
acetic acid as it is very hazardous in case of skin contact (irritant), of eye
contact (irritant), of ingestion or inhalation.
Adapted from chemical education journal 2001
Set Up Diagrams:
Steel wool in test tube
Procedure:
1) Fill a beaker or trough about ¾ full
2) Hold steel wool with forceps and rinse with acetone in fume cupboard, shake
off any excess (this removes any excess oils). Then rinse the steel wool in acetic
acid (this helps to speed up the rusting process).
3) Pull apart the steel wool to increase its surface area and insert into the test
tube, pushing it towards the end
4) Invert the test tube into the beaker of water and leave it there. Rest the mouth
of the graduated cylinder on the bottom of the beaker and lean the cylinder
against the side of the beaker as shown
Adapted from chemical education journal 2001
5) After 5 minutes move the test tube so the water level inside it is equal to the
water inside the beaker. Measure and record the height of the water in the test
tube and rest it on the bottom again
6) Measure and record the height of the water every 5 minutes as in step 5 until
the level stops changing
7) Measure and record the total height of the test tube. Calculate the percentage
of the final height of water in the test tube.
Use your results to determine the overall change in water height and thus
the percentage of oxygen in air
Sample Results/Observations
Time
Height (mm)
0mins
20mm
5mins
28mm
10mins
33mm
15mins
33mm
20mins
33mm
25mins
33mm
Height of Test tube: 140mm
Observations: There was a noticeable reaction taking place. This could be
observed from the colour change of the steel wool from silver/grey to reddish
brown in certain parts.
Also the water level noticeably changed, indicating a reaction took place
Calculations
%O2= constant water level reading (mm)
Tube length (mm)
% O2= 35mm
140mm
Adapted from chemical education journal 2001
23.57%
Height of Water over Time
35
30
MM
25
20
Height mm
15
10
5
0
0
25
10
15
20
25
Time in Minutes
Conclusion: A reaction between the steel wool and oxygen occurred, resulting in
the consumption of oxygen and the rise of the water level in the test tube. The
result of 23.57% do not compare with the accepted value of 20.833%. The water
level was higher than the expected literature value for percentage oxygen in air.
This may have been caused by rinsing the steel wool for too long in the acetic
acid. Too much acid can result in the formation of H2, changing the gas
composition/pressure in the tube; therefore giving inaccurate results
Possible Questions to Ask:
Write an equation for the reaction that is occurring
What happened to the water level in the tube? What caused this?
Before measuring why did you move the test tube so that the water levels inside
and out were the same?
How would you test for the presence of oxygen in the test tube once the water
level has stopped changing? What result would you expect?
Report your values for the percent oxygen in air, as well as the class mean
How does your value compare to the class value and the accepted value
(20.833%)?
If you had done this experiment at the top of Mt. Everest, would the results be the
same? Explain your answer.
Adapted from chemical education journal 2001
Links to Irish Curriculum: This links to the Irish curriculum very well. The
experiment could link in with atmospheric chemistry which is part of the leaving
certificate chemistry syllabus. It could act as an alternative method for
determining the percentage of oxygen in air and could make for an interesting
comparison with the other methods in terms of reproducibility, reliability and
accuracy.
This experiment could also be used effectively in the study of oxidation reactions
as it shows the oxidation of iron to form rust (FeO3). It shows the consumption of
oxygen in the reaction and thus provides a visual to accompany the theory on
oxidation and reduction.
Rusting is also an element of STS (science and technology in society). The
effects of rusting, its prevention etc could be discussed with reference to this
activity
More information
On rusting
http://www.sciencetechnologyaction.com/lessons/51/IMDO.pdf
On finding the percentage oxygen in air
http://www.bbc.co.uk/schools/gcsebitesize/chemistry/changestoearthandatmosph
ere/2earthsatmosphererev3.shtml
Adapted from chemical education journal 2001