edited bv ROBERTREEVES
filtrate/ & re~idue~
Marlborough School 250 S Rossmore Avenue Los Angeles. CA 90004
Percent Oxygen in Air George F. Martins Newton North High School, Newtonville, MA 02160
Gases make up one of the standard topics in high school chemistry, and several experiments on gases are done in the student laboratory. These studies include the laws of Boyle and Charles, the general gas law applied to molar volume or reaction quantities (with the partial pressure law), and effusion rate studies. The determination of oxygen in air is not a common ex~eriment.althouah in sonhisticated form i t is performed in the college laboratory. The composition of air ( ~ e r c e n oxveen t hv volume) is a useful evervdav . "tonic . in rklation to poliution and acid rain discussions, and the relative auantitv of oxvaen in air c o m ~ a r e dto that in water gives t h e student an awareness of the serious prqblem of oxygen depletion in water pollution. T h e following description is a modification of a simplified determination used in the college laboratory.' The experiment, the reaction of iron with oxygen in air, can he performed with prelah discussion in a standard period, and with postlab discussion and student work on a report during a double period. With variations the experiment is suitable for student levels ranging from descriptive to honors chemistry. Equlpment and Materials
The high school chemistry teacher will hardly find a simpler experiment than the reaction of iron with oxygen in air. The reaction vessel is a standard test tube 16-20 mm by 150 mm with a 15-cm metric rule attached hv a small rubber band. Size 00 steel wool procured from the local hardware store is the reactina iron and a solution of 0.5 M acetic acid is the conditioner fo;the steel wool. Time may he read on the classroom clock. About 1.0 g of iron is ideal. The test tube assembly with steel wool is inverted in a 400- or 600-mL beaker two-thirds full of water.
re-Lab Dlscusslon Students are shown the apparatus and materials and are asked to develop a quantitative measure from experiment of the percent oxvaen in air assumine that iron rusts bv reacting k i t h oxygen. Discussion leads-to the proportionality of length and volume of the tuhe and the Dercentaae calculation.
and then quickly invert the assembly in the beaker just below the water level noting the time. Adjust, if necessary, the 0 rnm ruler mark to the water level inside the tuhe. 4. At 2-min intervals record the time and position of the rising water level (top of meniscus) as oxygen is removed by reaction with iron. Lower the tube when making readings to maintain inside and outside water levels about the same. After 15 min, 5-min intervals may be suitable. Take two or three readings at final constant level. 5. While recording tabular data, construct a graph with the Yaxis from O to 30 mm water level and X-axis from O to 40 min, and insert points during measurements. 6. After completing the experiment, remove the wool with a wire hook, record its color, discard it, and clean the tuhe. Postlab Assignment
The ex~erimentalorocedure is reviewed. and individual team dataare inspected. A sample percent oxygen by volume is calculated so that all teams submit the percent values for class tabulation and statistical calculation. The following questions become part of the individual lab report. 1. Calculate the percent volume of oxygen in air, and record it on the class summary. Find the class average, and compare your result with it. 2. Assume that the iron reacts with oxygen in air to form iron(II1) oxide. Write an equation for the reaction. How does the product differ from the reactants, iron and oxygen? 3. Water at 20 "C contains 0.0092 g of oxygen per liter (1000g). Calculate the percent oxygen hy mass in water. How are you "luckier" than fish?
Sample Data and Results
Figure 1displays a student-pair data graph, and the table shows a summaw of a class's results. Combined results for three classes of 27 student teams gave an overall mean of 2070, which compares favorably with the accepted value of Constant water level is reached in 20 to 30 minutes. Major source of error is the positioning of the rule and reading to the nearest millimeter. Failure to spread out the steel wool may give long reaction times. Varlatlons for Student Dinerences
For students of descrintive chemist^. where mathemnti~ ~ ~ ~ cal considerations are not emphasized, the followingqualitative activitv mav he added. Add the final steel wool to dilute ~ gas in another test tube, and test the gas HCI c o ~ l e c &the with a match or alowina splint. Write eauations for the production of the gas a n d i t s reaction in t h e flame. For students of standard college DreDaratorv chemistrv . the following questions are offerei. ~
ronsranr lrvcl reading . . imm) X 100 O' = tube length tmm, Procedure 1. Measure the length in millimeters of a regular test tuhe. Use
the half point between the end and beginning of rounded end as the end of the tuhe. Attach the metric rule by a rubber band so that the metric length begins at the lip of the tuhe. 2. Remove a piece of steel wool of about 1 g from the pad, and mass to 0.01 g. Immerse the steel wool in about 20 mL of 0.5M acetic acid in a small beaker for 1 min. 3. Remove the steel wool from the acetic acid, and press to remove excess acid (rinse hands afterward). Pull the wool apart to increase its surface, insert halfway up the tube with a stirring rod,
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1. Use the equation for the reaction of iron with oxygen to calculate the total mass of oxygen needed to react all the iron used.
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Birk, J. P.; McGrath, L.; Gunter, S. K. J. Chem. Educ. 1981, 58, 804.
Barrow, Gordon M. GeneralChemistry;Wadsworth: Belmont, CA. 1972: p 9.
Volume 64
Number 9
September 1987
809
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Class Dab and Results
Time, min Plot of water level vs. time tor team no. 1
2. Calculate percent difference between your result and that of the dass average. Explain the comparison by reference to errors in your procedure.
For accelerated or honors students several variations are possible.
810
Journal of Chemical Education
team
iron
no.
9
Wbe mm
level max mm
time
max min
% oxygen
1. Graph on the Y-axis the percent ratio of water level reading to tuhe length. 2. ~ h density ; of steel is 7.9 g1mL. What percent of the tube volume was occupied by the wool? Use your graduated cylinder and water to measure the tube volume. Is the volume of wool a source of significant error? 3. Which percent oxygen, volume or mass, is greater? Show by calculation. 4. Give a hypothesis for the rinsing in acetic acid to reduce the reaction time from about 5 hours toless than 30 minutes. (Note: this is a research question because the answer far the catalysis is not known at present.) 5. Calculate the standard deviation for the class results and comment on the reproducibility of the experiment results.
