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.

19. Suppose that the actual O3 concentration at 12:30 p.m. at Site C was 43 ppb. Based on Figures 1 and 3, which of the following expressions would give the percent error for the value of the O3 concentration that was determined at 12:30 p.m. at Site C ?

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

According to common sense, percent error=|accepted value - experimental value| \ accepted value × 100%;

The 43 ppm given in the question stem should be the accepted value;

For the experimental value, see figure 3. At 12:30 pm, the I3- concentration of Site C is 1.0, which corresponds to figure 1. The O3 concentration at this time should be 40 ppm;

So the percent erro should be It is (43-40)/43, so option A conforms