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.

16. Based on Figures 1 and 3, the O3 concentration at 5:00 p.m. at Site B was approximately how many times as great as the O3 concentration at 2:00 p.m. at Site C ?

Answer and Explanation

Your Answer is

Correct Answer is G

Explanation

Looking at figure 3, the I3- concentration of Site B at 5 pm is about 2.0, back to figure 1, when the ordinate is 2.0, the abscissa O3 concentration is about 80;

And Site C The I3- concentration at 2:00 pm is about 1.0, back to figure 1, when the ordinate is 1.0, the abscissa O3 concentration is about 40;

So the O3 concentration of Site B at 5 pm About twice that of Site C at 2:00 pm