Question 37 Answer and Explanation _ACT December 2011 Form 70A Science Section

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 △Pvalue 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 CaCO₃content (percent of the total mass that is CaCO₃) for each substance using the equation below:

The results are shown in Table 1.

37. A student claimed that, in Experiment 1, increasing the amount of CaCO₃ that was added to the acid solution increased the reaction rate. Does Figure 1 support this claim?

A. Yes, because the time to reach the maximum pressure was shorter in the trial for 0.05 g than it was in the trial for 0.01 g.

B. Yes, because the time to reach the maximum pressure was longer in the trial for 0.05 g than it was in the trial for 0.01 g.

C. No, because the time to reach the maximum pressure was shorter in the trial for 0.05 g than it was in the trial for 0.01 g.

D. No, because the time to reach the maximum pressure was longer in the trial for 0.05 g than it was in the trial for 0.01 g.

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

Look at figure 1, when the mass of CaCO₃ is greater, the maximum pressure will be reached first, that is, the reaction speed will be faster. Figure 1 can confirm the statement in the stem