Passage VII

Calcium carbonate (CaCO3) is the primary component of chalk, eggshell, and other substances. When CaCO3 reacts with an acid, CO2 gas is produced:

CaCO3 + 2H+ Ca2+ + CO2 + H2O

Students did 2 experiments to determine the CaCO3 content of several common substances.

Experiment 1

A flask was fitted with an airtight cap. The cap contained a pressure sensor connected to a computer. Next, 30 mL of acid solution (a volume sufficient to react with up to 0.5 g of CaCO3) was added to the flask. Then, 0.01 g of powdered CaCO3 was added to the solution. The flask was quickly recapped, and pressure readings (in kilopascals, kPa) were recorded every 0.1 sec for 14 sec. The procedure was repeated using different masses of powdered CaCO3 (see Figure 1).

For each trial, the total change in pressure (△P) was calculated using the equation below:

△P = (maximum pressure) - (initial pressure)

The △P value for each trial was then plotted against the mass of CaCO3 added in that trial and a best-fit line was drawn through the points (see Figure 2).

Figures adapted from M. M. F. Choi and P. S. Wong, "Using a Data-logger to Determine First Order Kinetics and Calcium Carbonate in Eggshells." ©2004 by Division of Chemical Education, Inc., American Chemical Society.

Experiment 2

Students used the procedure from Experiment 1 to test 0.05 g samples of powders made from antacid tablets, calcium supplement tablets, and other substances. After calculating the △P value for each trial, they used the best-fit line in Figure 2 to convert each △P value to a mass of CaCO3, and then calculated the CaCO3 content (percent of the total mass that is CaCO3) for each substance using the equation below:

The results are shown in Table 1.

35. A 0.05 g sample of powdered seashell was tested as in Experiment 2. The CaCO3 content was calculated to be 81%. Based on Figure 2 and Table 1, △P for the seashell sample was most likely:

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

As can be seen from figure 2, the higher the CaCO3 content, the larger the ΔP should be. The CaCO3 content in the question stem is 81%, which is less than 87% in table 1, so △P is also less than 25.2, and the 21-26 kPa in the options should be closer