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

The results are shown in Table 1.