Percent Composition of Calcium Carbonate in Tums‚ a Chemistry Laboratory Experiment.
Bruce Mattson and Emily Saunders, Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA
website: http://mattson.creighton.edu/Microscale_Gas_Chemistry.html
Overview
This is a classroom group laboratory experiment that utilizes everyone’s data to give an overall result that demonstrates the
concept of percent composition.
Background
In this experiment a unique method of analysis is used that utilizes the volume of carbon dioxide generated by the acid
decomposition of calcium carbonate samples inside a syringe. In Part 1 of this experiment students produce a graph as a
group effort that shows the relationship between the mass of pure calcium carbonate used and the volume of carbon dioxide
produced. The reaction is:
CaCO2(s) + 2 HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
The group graph produced should look similar to Figure 1 (data provided at the end). In Part 2 of the experiment, students
react samples of Tums of known mass as they did with the pure calcium carbonate. The volume of carbon dioxide they
obtain can be converted to mass of calcium carbonate with the use of the graph. Knowing the mass of the sample of Tums
used and the mass of calcium carbonate from the graph, they can determine the percent calcium carbonate in Tums.
Figure 1. Volume of carbon dioxide produced as a function of mass of calcium carbonate used.
Instructions
Accurately reading the volume gradations on the syringe
The volume of the liquid level inside the syringe is generally easy to read because water does not exhibit a meniscus with
plastic as it does with glass. Nevertheless, two common sources of error must be avoided. The syringe must be perfectly
vertical in order for an accurate reading to take place. We set the syringe balancing on its syringe cap on a flat surface. Read
the syringe with eyes at the same level as the liquid. It is possible to estimate the volume to within + 0.2 mL. The vial cap
will cause erroneous readings if it is floating near the calibration marks. To read the volume near the black rubber seal, we
recommend reading the position where the seal first comes in contact with the barrel from the perspective of inside the
syringe. It is possible to estimate the volume to within + 0.3 mL.
Part 1. Class Calibration Curve
1. Using a top-loading balance, measure into a weighing dish the quantity of calcium carbonate that you and your lab partner
were assigned to use.
2. Place an empty vial cap on the analytical balance and tare the balance to read 0.0000 g. Remove the vial cap and carefully
transfer the pre-measured CaCO3(s) into the vial cap. Do not get any of the solid on the outside of the vial cap. Return the
vial cap to the analytical balance and determine its exact mass. Record the exact mass on your Report Sheet.
3. Lower the cap containing the CaCO3(s) into the syringe by flotation.
4. Measure out 6 - 8 mL 3 M HCl(aq) into a weighing dish.
5. Draw up 5 mL of the acid into the syringe. Push the syringe fitting into the syringe cap. Use caution so that the reagents
do not mix until Step 7.
