Concept/Skills Development
LABORATORY ACTIVITY:
STUDENT VERSION
Activity 2: Oxygen in the Air
Purpose
To measure the volume percentage of oxygen in moist air and calculate from it the percentage of oxygen in dry air.
Safety
1. Wear protective goggles throughout the laboratory activity.
2. All chemicals should be treated with respect&emdash;even the most common are not exceptions. Be sure to wash off any chemical with water as soon as it contacts your skin.
3. Avoid breathing acetone vapors or spilling it on your skin.
4. Take care not to cut yourself while handling steel wool.
5. Washing of steel wool should be done only in the fume hood, using forceps.
6. Acetone is flammable; keep it away from flames.
Procedure
1. To each of two 20- x 150-mm test-tubes, attach lengthwise a strip of masking tape on which to record water levels. Set up a ringstand and two utility clamps, so that the inverted tubes can be set up in the 1000-mL beaker as shown in Figure 4. The beaker should be 2/3 full of water.
2. Weigh two l.00 g pieces of size 00 steel wool, taking care not to compact the material.
3. In the hood, rinse in acetone (for about 30 s) one piece of steel wool, using small forceps. This removes any oil from the surface. Shake off any excess and drain on a paper towel in the hood. Then place the steel wool in the 25.0 mL of l M acetic acid provided in the hood and agitate for about 1 min. Shake off and drain as before on another paper towel.
4. Again rinse the steel wool, this time using 25.0 mL of 0.1 M acetic acid. (Agitate for 30 s, shake off excess and drain again on a fresh paper towel.)
5. Insert the cleaned steel wool into a test-tube, pushing it to
the bottom with a stirring rod without compressing it. It should
spread over the bottom half of the tube as shown in Figure 4.
Immediately invert the test-tube and carefully lower it into the
beaker of water. Tilt the tube and beaker so that approximately 2.0
cm of water goes up into the tube. Adjust the levels of water until
they are the same inside and outside the tube, and clamp the
test-tube in a vertical position.
6. Mark the initial water level on your tape with a waterproof pencil or pen (A in Figure 5).
7. Repeat the entire procedure (Steps 3-6) for the other tube, taking care not to disturb Tube 1 as you set up Tube 2. The mouth of the tubes should be below water level throughout the activity.
8. When no further change in water level can be detected, after about 20-30 min (it is acceptable to leave the set-up overnight at this point), wait 5 min longer and then, by lowering the tube, readjust the water levels inside and outside until they are the same.
9. Mark the water level carefully on your tape (B in Figure 5), take an accurate temperature reading of the water in the beaker without touching the sides (we will assume the gas inside the tube and the water in the beaker are the same temperature), and a barometric pressure reading. Construct a data table in your laboratory notebook and record your data.
10. Drain the first test-tube, dry it, and accurately estimate the distance between the two lines to the nearest hundredth of a centimeter. Next, estimate the length from the initial water level to the bottom of the tube (A-C distance in Figure 5) to the nearest hundredth of a centimeter. Using these data calculate the percent by volume of O 2 in moist air.
11. Correct your percent value to that of dry O 2 using the following equation: where P b is the barometric pressure, and P w is the water vapor pressure at the temperature recorded.
12. Thoroughly wash your hands before leaving the laboratory.
Data Analysis and Concept Development
1. Why are two test-tubes used in Step 1?
2. Why must you be careful not to pack the steel wool in Step 2?
3. Why is it necessary to rinse the steel wool in acetone and acetic acid?
4. What is the purpose of equalizing water levels in Step 5?
5. Why did water rise into the tubes in Step 8?
6. Why is it necessary to adjust water levels again in Step 8?
7. Why do you need to know gas temperature and barometric pressure?
8. What equation did you use for the calculation of percent by volume of O 2 in moist air in Step 10?
9. Why is the air moist?
10. How does your corrected percentage of dry oxygen in air calculated in Step 11 compare with the accepted value of 20.9% volume O 2 for dry air at sea level? Calculate your percentage error.
Implications and Applications
1. In geology, iron deposits can be used to hypothesize atmospheric change and the evolution of life producing that change by the state of oxidation of the iron. For instance, in what order would you expect to find banded layers of FeO and Fe 2 O 3 in deep sediments as you dig into the crust? What does this imply?
2. Ozone (O 3 ), an allotropic form of oxygen, plays a major role in screening harmful ultraviolet light from sunlight in the atmosphere. What implication is there for the change in banded iron deposits and the development of the ozone layer around the earth?
3. If you were to test the remaining gas in each of the test-tubes used in this activity with a burning splint, what would occur? Why? What gas would this be if all but a very tiny fraction reacted with magnesium to form Mg 3 N 2 ?
4. Based on conclusions in Question 3, what can you imply about combustion and/or respiration in the atmosphere compared to that in a pure oxygen environment? Is it not fortuitous (a “lucky accident”) that 78% of the atmosphere is composed of this “remaining gas”? Give an example to illustrate what might happen if it were not.
5. How would you design an activity to measure the percentage loss of O 2 and percentage gain of CO 2 in respired air?
6. Slow oxidation (rusting) of iron is a major economic problem. A person who found a “better way” to prevent or slow the process would probably become a billionaire overnight. Since this is an oxidation-reduction process, what are three methods currently used to retard the process?
LABORATORY ACTIVITY:TEACHER NOTES
Activity 2: Oxygen in the Air
Major Chemical Concept
Earth’s atmosphere is very different from the planet’s original one composed chiefly of methane (CH4 ) and ammonia (NH3 ), with oxygen estimated to have been only 10 –12 what it is today. Although the earliest oxygen produced seems to have been used to oxidize reduced atmospheric gases like CH 4 , NH 3 , H 2 S, etc. and crustal materials, like the production of banded iron formations from iron rising from the core, photoautotrophs gradually increased the percentage of O 2 in the atmosphere over millennia. Today, there is an approximate “reservoir” of 10 15 tons of atmospheric oxygen. Theoretically, all of the O 2 produced by plants should have been used by them in respiration. However, since some plant material produced became geologically “encapsulated” in the crust as fossil fuels and other organic substances, a surplus of oxygen gradually built up in the atmosphere. The student should gain an insight into the important role O 2 plays and has played in the development of life on this planet. In addition the reactive nature of the element should be demonstrated, while a reasonably accurate quantitative determination of O2 in today’s atmosphere can be made using simple techniques. Also, a related discussion of ozone and its importance can be brought in as a related topic.
Level
This activity is appropriate for any general first year chemistry class.
Expected Student Background
Students should have studied the preparation and properties of oxygen and the gas laws. Particularly, they should know Dalton’s Law of Partial Pressures and how to adjust for water vapor pressure. They will probably have studied the atmosphere in earth science, but this will be an effective way to verify previously learned facts.
Time
This activity can be done in one 50-min period, but it will be most effective if several hours are allowed for the reaction. Other strategies are to have students set up the activity after school the day before the activity, or set up in the laboratory and come back after school to take final readings and clean up. Set up takes about 20 min. Discussion after the activity may require another period.
Safety
Read the Safety Considerations in the Student Version.
1. Be sure the washing of the steel wool in acetone is done in a fume hood.
2. Students should avoid breathing of acetone vapors or contact with skin, and no burners should be in use nearby, since acetone is flammable.
3. Beakers containing 100 mL of acetone and l.0 M acetic acid should be set up in the hood for washing the steel wool. You should change these after use by two or three students. A reagent bottle of 0.10 M acetic acid should be available for students to obtain the 25.0 mL needed for their final rinse for both pieces of steel wool. Dispose of used acetone properly.
4. Students should be warned that “tearing” pieces off of steel wool can shred the skin painfully.
5. Acetone should be allowed to evaporate in the hood.
Materials (For 24 students working in pairs)
Nonconsumables
12 Beakers, 100-mL
12 Beakers, 1000-mL 24 Test-tubes, 20- x 150-mm [Note: Using 50-mL burets (if available) in place of test-tubes will improve volume measurement.]
24 Utility clamps
12 Ringstands
12 Rubber stoppers, #2 (optional)
12 Waterproof pencils or pens
12 Stirring rods
Consumables
Acetone, l L l.0 M
Acetic acid, CH 3 COOH, 1 L (57 mL of 17.5 M [glacial] acetic acid diluted to l.0 L). REMEMBER: Add acid to water.
0.10 M Acetic acid, CH 3 COOH, 1 L (5.7 mL of 17.5 M acetic acid diluted to l.0 L)
Fine steel wool (size 00),
1 pkg Wood splints
Masking tape
Advance Preparation
Prepare needed acetic acid solutions and secure other materials for the activity.
Pre-Laboratory Discussion
Discuss safety with students. Emphasize the importance of not compressing the steel wool and accurate marking of tape with a waterproof pencil or pen. A small amount of water should be in the test-tube when it is set up as a starting point. Tilting the tube in the beaker allows a few milliliters of water to replace air in the tube. Care should be taken not to remove the test-tubes from the beaker until all water levels have been adjusted and accurate markings made. Also mention that the reason water levels must be equalized is that the total gas pressure inside the tube is the same as the student reads on the barometer. Inform students where to find a table of equilibrium water vapor pressure values. If students are to do the optional splint test for oxygen, remind them to stopper tubes before removal from the beaker. Finally, waste steel wool should be disposed of properly, not in the sinks, and acetone should be evaporated in the hood. Warn students that there may be some rust staining in their tubes, which can be removed with oxalic acid solution.
Teacher-Student Interaction
Students should be monitored at the hood to insure proper washing of steel wool and handling of acetone. Walk around and make sure students allow a small amount of water into the tube before clamping. Also insure that tubes are in a vertical position and that the starting position is marked after set up.
Anticipated Student Results
Students should be able to obtain a percent dry oxygen in air within 10% of the accepted value (20.9%).
Answers to Data Analysis and Concept Development
1. Duplicate set-up in case of misfortune.
2. Since O 2 reacts at the surface of the steel wool, and since it is important that all O 2 in the tube react, we do not want to limit the amount of iron exposed to the air in the tube by compressing the steel wool.
3. To insure that the steel wool is clean and free of oil and corrosion. Acetone will dissolve oil. Acetic acid will insure a clean surface.
4. Equalization is done to insure that the gas pressure inside the tube is known and, therefore, is the same as the barometric pressure.
5. As O 2 was removed from the air, the total gas pressure in the tube decreased, allowing the greater air pressure to force water into the tube until pressures inside and outside equilibrated.
6. Same answer as for Question 4.
7. The temperature is necessary to obtain the equilibrium water vapor pressure in order to adjust total pressure. The barometric pressure is equal to the total pressure inside the tube when the water levels are the same.
8. Volume % moist O 2 = Length water level rose Total length of original air column x 100%
9. The air is moist because it was over water, and some water vapor was present.
10. Answers will vary, and they should be less than 10% error.
Answers to Implications and Applications
1. Closer to the surface you would expect to find Fe 2 O 3 , with FeO deposits buried underneath these layers. This implies that the early atmosphere contained less oxygen and there was only enough to oxidize the crustal iron to the 2+ state, while later sediments indicate a greater abundance of oxygen, allowing oxidation to the higher 3+ state.
2. The increase in oxygen in the atmosphere indicated by the banded iron deposits would imply a concurrent increase in ozone production, resulting in the formation of a significant ozone layer around the earth: 3O 2 (g) 2O 3 (g) Since this layer would increasingly screen harmful ultraviolet wavelengths from sunlight, life could begin to move out from the ocean depths into the direct sunlight on land.
3. The burning splint would immediately go out, indicating that the gas(es) remaining after all of the oxygen has been removed do not burn or support combustion (respiration). The formation of magnesium nitride indicates that most of the remaining gas was nitrogen.
4. An object should burn much faster in pure oxygen than in air, since air is only approximately 20% oxygen (see Reaction Rates module). If each lungful of “air” were 100% oxygen, we would literally hyperventilate ourselves to death, since the cells would probably not be able to withstand the temperatures produced or resist being oxidized themselves by rapid reaction.
5. For O 2 : Using the set-up in Activity 2, substitute a tube of respired air and perform the experiment using the same procedures, making sure that temperature and pressure conditions are the same as when air alone was used. The average weight-gain difference divided by the weight-gain of the iron in air times 100% gives the percent loss of oxygen in respired air. (The design of an apparatus containing only respired air would require a two-hole stopper and inlet and outlet tubes to the test-tube.) For CO 2 : Invert equal volumes of air and respired air at the same temperature and pressure conditions in a container of limewater. After standing, and the formation of a precipitate, note any difference in water levels in the two containers. This difference could be used to find the percent volume difference of CO 2 in the two samples by dividing the difference by the volume of CO 2 removed from air and multiplying by 100%. Anything over 100% indicates the percent increase of CO 2 in respired air. Obviously several trials to obtain averages and large enough samples to produce measurable results must be used.
6. Three methods are: (1) lubrication, to exclude oxygen; (2) painting, also to exclude oxygen; and (3) use of a more reactive (than iron) metal to act in a sacrificial capacity and undergo oxidation in the redox process of corrosion. Other methods that prevent oxidation of the iron may be mentioned.
Post-Laboratory Discussion
Use Implications and Applications as a basis for discussion. Ask students if this reaction would have occurred without water being present. Then ask students to balance the equation: Fe(s) + O 2 (g) + H 2 O(l) ® Fe 2 O 3 . x H 2 O(s). (The x moles of water appearing on both sides of the equation indicates that iron oxide hydrate can have a variable amount of water.) Discussion of the stoichiometry of photosynthesis and respiration is also appropriate here: why there should be no surplus of oxygen in the atmosphere, and what the fact that one exists implies. This fact can also be used to illustrate the living (bio-) and nonliving (geo-) components of the oxygen cycle, their interrelationships, and how important the living portion of the cycle (rain forests, phytoplankton, etc.) are to it. Ozone and its depletion is another way to point out possible consequences of human intervention in the natural cycles on earth. “Spaceship Earth” has many backup systems to try to compensate for “system breakdowns.” What happens when these backups fail due to human activities can seriously impact natural systems. If time permits, possible sources of error might be discussed.
Assessing Laboratory Learning
Percentage volume of dry oxygen in air should be 20.9%. Since this activity is intended to give an approximate percent volume, a 10% error is acceptable. Students should be required to explain possible sources of error that might cause too high or too low readings.
CAUTION: Use appropriate safety guidelines in performing demonstrations.