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
Download pdf of Percent Composition  from our book Microscale Gas Chemistry

Go to Microscale Gas Chemistry Home Page

This article first appeared in Chem13 News, 313, September, 2003

Overview.
    This classroom group laboratory experiment utilizes everyone’s data to give an overall result that demonstrates the concept of percent composition.  A second, more traditional method is also described so that the results can be compared.  Each experiment costs only pennies to do and the experimental part can be completed within a 40-minute laboratory period.

Background.
    There are dozens of laboratory manuals that include an experiment with a title similar to that of this article.  One common method entails reacting the antacid with excess acid and back-titrating with base.²  In another approach, Alan Slater and Geoff Rayner-Canham describe a microscale experiment³ in which the calcium carbonate content of an antacid is determined by three mass measurements: (a) the mass of the sample of crushed Tums; (b) the mass of the Tums sample plus the mass of a small quantity of 4 M HCl(aq) in a separate container; and (c) the mass of Tums + acid after they have been mixed and the reaction has taken place.  The loss of mass is attributed to carbon dioxide.  From there, the percent CaCO3 in the Tums can be determined.

Description.
    In this article we describe a unique method of analysis that utilizes the volume of carbon dioxide generated by the acid decomposition of calcium carbonate samples inside the 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:

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.

    In Part 3, students perform the experiment using the general method of Slater and Rayner-Canham:   They determine the mass of carbon dioxide lost during the reaction of Tums with HCl(aq), convert mass of CO2 first to moles of CO2, then to moles of CaCO3 and finally to mass of CaCO3.  With that, they can determine percent CaCO3 as in Part 2.

    Analyses of Tums tablets by these two methods give similar results.  In several trials, we have determined by the syringe method described (Part 2) that Tums are 36 - 39% CaCO3.  Using the traditional method (Part 3), we get 41 - 43% CaCO3.  We think two factors contribute to the discrepancy: (1) CO2 is soluble in water so that the syringe method gives slightly low results and (2) The Tums tablets splatter water as they react with considerable fizzing; some water is undoubtedly lost this way in the traditional method, giving results that are slightly high.  Nevertheless, the results are similar.
 

Materials and Chemicals needed:⁴

• 60-mL plastic syringes   • Latex syringe caps  • vial caps
• top-lading balance  • analytical balance  • weighing dish
• spatula     • 4-L pail (plastic ice cream pail)
• 3.0 M HCl   • CaCO3(s)   • Tums tablets

Instructions:
    Before students arrive:  Each experiment will consume 5 - 7 mL 3 M HCl(aq), however, we suggest that the volume estimation be based on 8 ? 10 mL per experiment.  Prepare a bottle of approximately 3 M HCl(aq) by diluting 45 mL concentrated HCl with 135 mL water to give a total of 180 mL (or diluting 90 mL 6 M with 90 mL H2O).  Determine which pair of students will perform each experiment.  Include the following approximate masses of pure calcium carbonate in the list of experiments to be done: 0.04 g, 0.05 g, 0.06 g, 0.07 g, 0.08 g, 0.09 g, 0.10 g, 0.11 g, 0.12 g, 0.13 g, 0.14 g, and 0.15 g.  Hint: Make an inexpensive “spill-proof” spatula for each pair of students by cutting the bulb off of a pipet at an angle:
 
 

 Vapor pressure of water.
    Adjusting for the vapor pressure of water is unnecessary because both Parts 1 and 2 are both performed in the same way and presumably at the same temperature.  Technically, the graph produced in Part 1 should have the volume of CO2 (y-axis) adjusted to subtract the volume attributed to water vapor.  For example, at 20 oC, the vapor pressure of water is 17.5 mmHg, so if the external pressure were 740 mmHg at the time of the experiment, the adjusted volume of CO2 would be (740 ­ 17.5)/740 = 0.976 of the value determined using the student instructions below.  However, the volume of CO2 determined in the Tums analysis would have to be similarly adjusted, and the net result would be the same as if no compensation for water vapor pressure were included.

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.
 

Instructions for the Students.
    These instructions assume that students are familiar with the general procedure of gas preparation.
 

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 premeasured 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.

6. Read the initial volume of the syringe using the bottom of the rubber seal as the mark (Figure 3).  Also read the level of the acid solution.  The difference between these two readings is the volume of air in the syringe.  This volume will be subtracted later.  Record your data.

Figure 3. Reading the syringe.

7. Perform the reaction by shaking the syringe.  The reaction is fast.  Assist the plunger from time to time by pulling it outward by a few mL.  The reaction is done within a few seconds — when no more bubbles are being produced in the solution.

8.  (See a 1-minute movie demonstrating this step.) You are now ready to measure the final volume.  You’ll get your hands wet doing it!  First, pull the plunger outward until it feels like you pulling against a force.  Let go of the plunger and it will return to an “equilibrium” position where the pressure inside the syringe is fairly close to the outside pressure.  Carbon dioxide tends to dissolve in water (like it does in carbonated beverages), so we must force it out of solution before we measure the volume of CO2(g).  Here’s how:  Pull the plunger out by 10 mL or so from its equilibrium position and shake and tap the syringe vigorously.  Notice that your action forces dissolved carbon dioxide to come out of solution with noticeable bubbling.  Next, immediately remove the syringe cap underwater while holding the plunger outward creating a reduced pressure — use a large container such as an ice cream pail in order to accommodate your hands and the syringe.  Remove the syringe cap deep enough under enough water so that only water — no air — enters the syringe.  Water will rush into the syringe to equalize the pressure.  Recap the syringe underwater.  The gas pressure inside the syringe is now very close to the atmospheric pressure outside the syringe.  Be careful to not move the plunger inward or outward after it has been recapped.  Take the final volume readings for both gas and solution as previously done in Step 6.  The difference in volumes this time is the volume of carbon dioxide + air initially present.  The volume of carbon dioxide only is obtained by subtracting the volume of air (Step 6) from the volume of carbon dioxide + air just determined.  Record all results.

9. You instructor will provide you with instructions for sharing the data with your classmates (such as plotting your results on a group graph).
 

Part 2. CaCO3 in Tums tablet using class graph and volume of gas.

    Repeat the experiment (Steps 1 - 8) with a sample of Tums instead of pure calcium carbonate. Tums tablets consist of calcium carbonate and a number of other ingredients as listed on the bottle. Only calcium carbonate produces gas in the reaction with HCl(aq).  Use a mass of approximately 0.25 - 0.32 g.  The sample can be used as a chunk; it does not need to be pulverized.  Record the exact mass used.  Record the four volume readings as per Steps 6 and 8 above.  You will notice that the Tums does not react quite as quickly and leaves a milky solution after the evolution of gas has ceased.

Part 3. CaCO3 in Tums tablet using mass lost method.

1. Place 15 mL 3 M HCl(aq) in a 250 mL plastic cup.

2. Determine the mass of a Tums tablet. (It will be approximately 1.3 ? 1.4 g)

3. Place the cup of acid and the Tums tablet side by side on the balance and determine the total mass.

4. Remove the acid and Tums from the balance and add the Tums tablet to the acid.  It will fizz as it releases CO2(g).  Wait until the bubbles have stopped being formed.  Swirling the cup will accelerate this process.  Because CO2(g) is heavier than air, tip the cup slightly to “pour out” the CO2(g), however, do not pour out any liquid.

5. Determine the mass of the resulting solution.  The difference in mass between this and the mass determined in Step 3 is due to the CO2(g) lost during the reaction.  This can be converted into moles of CO2(g).  This also equals the moles of CaCO3(s) — see equation above.  One can then convert moles of CaCO3(s) into mass of CaCO3(s) and determine the % CaCO3 in the Tums tablet.

Laboratory Report:
Part 1. Class Calibration Curve

Mass of pure CaCO3(s) used:
Volume of carbon dioxide calculation:
Initial syringe readings:
Rubber seal (mL):  Solution (mL):     Volume air (mL):
Final syringe readings:
Rubber seal (mL):  Solution (mL):     Volume air + CO2 (mL):
Volume of CO2 collected (mL):

Mass of Tums used:
Volume of carbon dioxide calculation:
Initial syringe readings:
Rubber seal (mL):  Solution (mL):     Volume air (mL):
Final syringe readings:
Rubber seal (mL):  Solution (mL):     Volume air + CO2 (mL):
Volume of CO2 collected (mL):

Mass of Tums used:
Mass of cup of acid + Tums tablet before reaction (Step 3):
Mass of cup of acid + Tums tablet after reaction (Step 5):
Mass of CO2 produced (g):

Part 2. CaCO3 in Tums tablet using class graph and volume of gas.
    1. What mass of CaCO3(s) was present in your Tums sample?  After all of the data from all groups are available, you can now answer this question as described below.  Your teacher will provide you with either (a) or (b):

(a) A sketched line on the class graph for direct use.  In this case, you should locate on the graph’s y-axis the volume of CO2(g) produced in your Tums experiment.  (i) Draw a horizontal line that intercepts the sketched line and (ii) drop a vertical line to the x-axis.  (iii) This is the mass of CaCO3(s) in your Tums sample.  In the example below, the mass of CaCO3(s) turns out to be 0.113 g.

(b) The equation for the line in the form of y = mx + b.  The slope, m =  Dy/Dx where y = volume of CO2 and x = mass of CaCO3; can be estimated or calculate using a spreadsheet such as Excel.  The y-intercept, b, should be 0.  If your teacher provides you with the equation, it will look like (using the example above):

volume of CO2 = 238 X mass of CaCO3 where the slope, with units, is 238 mL CO2/g CaCO3

Rearrange to solve for mass of CaCO3:

mass of CaCO3= volume of CO2 / 238 mL CO2/(g CaCO3)^-1

Part 3. CaCO3 in Tums tablet using mass lost.

1. What is the mass of CO2(g) produced from the reaction?

2. How many moles of CO2(g) were produced?

3. How many moles of CaCO3(s) must have been present to produce this amount of CO2(g)?

4. What mass of CaCO3(s) must have been present?

5. What is the percent CaCO3 in your Tums sample?

6. How do the results for %CaCO3 from the two methods compare?

7. Which method do you think gives better results?  What are possible sources of error in each method?

Website and books.
    It is possible to download this experimental procedure, laboratory report, and questions from our website⁵ as a Microsoft Word file (Word 2000 for PC and Word 98 for Mac).  The web-based document includes answers to the concept questions.  Numerous other experiments with gases are available at the website as well as in our books.^{6, 7}

Notes:

1.  Author to whom correspondence should be addressed.  E-mail: xenon@creighton.edu

2.  Examples: (a) “Analysis of an Antacid”, Working with Chemistry, Laboratory Separates, W. H. Freeman; (b) Titration: Standardization of a Base and Analysis of Stomach Antacid Tablets, D. A. Katz, http://www.chymist.com/

3. Microscale Chemistry Laboratory Manual, Alan Slater and Geoff Rayner-Canham, Addison-Wesley Publishers Ltd., 1994.

4.  This equipment and our two books can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), and Fisher Scientific.  Part numbers and links to their websites are provided at our microscale gas website.

5.  Website:  http://mattson.creighton.edu/Microscale_Gas_Chemistry.html

6. The Chemistry of Gases, A Microscale Approach, Mattson, B. M., Anderson, M. P., Schwennsen, Cece, Flinn Scientific, 1999, ISBN #1-877991-54-6.

7. Microscale Gas Chemistry, Mattson, B. M., Educational Innovations, 2000, ISBN #0-9701077-0-6.

This page last updated 26 Oct 2016