Passage III
When the ozone (O3) in air is mixed with an acidic solution of iodide ion (I-), the O3 reacts to form triiodide ion (I3-), O2, and H2O.
O3 + 3I-+ 2H+ → I3- + O2 + H2O
Students performed an experiment to determine the concentration of O3 in samples of air. Figure 1 shows the relationship between the concentration of I3-, in μmol/L, and the concentration of O3, in parts per billion (ppb), under the conditions of the experiment.
Experiment
At each of 4 outdoor sites, the students assembled the apparatus shown in Figure 2. First, they attached a long piece of tubing to a stand so that one end of the tubing was 1.5 m above the ground. Then, they placed the other end of the tubing into one hole of a 2-holed stopper. Next, they placed one end of a shorter piece of tubing into the second hole of the 2-holed stopper and attached the other end of the shorter tubing to a vacuum pump.
At each site, the students performed the following 2 steps every half hour from 12:30 p.m. to 5:30 p.m. on a particular day:
1. A 10.0 mL volume of an aqueous acidic solution having a 10 mg/mL concentration of I- was placed into a clean empty test tube that was then sealed with the 2-holed stopper.
2. The vacuum pump was turned on for 25 min to collect an air sample. Then, the pump was turned off and the concentration of I3-, in micromoles/liter (μmol/L), in the solution was measured.
The results are shown in Figure 3.
Figures adapted from J. V. seeley et al., "A Simple Method for Measuring Ground-Level Ozone in the Atmosphere." @2005 by the Division of Chemical Education, Inc., American Chemical Society.
17. At site A, was the O3 concentration highest at 4:00 p.m. or at 5:30 p.m. ?
Your Answer is
Correct Answer is C
Explanation
See figure 3, the I3- concentration of Site A at 4:00 pm is about 1.5, and the I3- concentration at 5:30 pm is about 2.4;
According to figure 1, The more I3-, the more O3, so the O3 concentration at 5:30 pm should be higher