Stoichiometry – Determination of Percent by Mass of NaHCO3 in Antacid Tablets
Introduction:
There are a number of over-the-counter antacid and pain relief medications that are taken by
being dissolved in water before they are ingested. Generally, an effervescent antacid tablet, such as
Alka Seltzer, contains aspirin (acetylsalicylic acid), citric acid, and sodium hydrogen carbonate (NaHCO3).
As soon as the tablet dissolves in water, an acid-base reaction involving sodium hydrogen carbonate
takes place. The generation of carbon dioxide from the acid-base reaction causes the bubbling. The
release of carbon dioxide into the atmosphere results in a mass loss after the reaction. With the mass
loss, one should be able to calculate the amount of sodium hydrogen carbonate reacted, and determine
the percent by mass of NaHCO3 contained in the antacid tablet. According to the balanced chemical
equation, the ratio of moles of NaHCO3 to moles of CO2 is one-to-one. In other words, the moles of
NaHCO3 reacted equals the number of moles of CO2 produced. Therefore,
mass of NaHCO3 reacted = (mass of CO2 generated/molar mass of CO2) x (1 mol NaHCO3/1 mol CO2) x
molar mass of NaHCO3
% by mass of NaHCO3 = (mass of NaHCO3/mass of antacid tablet) x 100%
HX (aq) + NaHCO3 (aq) → H2O (l) + CO2 (g) + NaX (aq)
HC2H3O2(aq) → H+(aq) + C2H3O2-(aq)
acetic acid (vinegar)
added to aqueous sodium produces
H3C6H5O7(aq) → 3 H+(aq) + C6H5O73-(aq) added to aqueous sodium produces
Citric acid
NaC2H3O2 (aq)
acetate
citrate
sodium acetate
Na3C2H3O2 (aq)
sodium citrate
Objective: To determine the percent of sodium hydrogen carbonate in antacid tablets
Materials: antacid tablets (original formula), vinegar (white, distilled, 5%), plastic cup, triple beam
balance, 50 mL and 10 mL graduated cylinders
Prodedure:
1. Determine the percentage of NaHCO3 in a tablet by dissolving it in water.
a. Add 35 mL of water to a clean plastic cup.
b. Measure and record the total mass of the cup with 35 mL of liquid in it.
c. Measure and record the mass of the antacid tablet.
d. Drop the antacid tablet into the cup and carefully swirl the cup to ensure complete
dissolution of the tablet and release of carbon dioxide.
e. Measure and record the mass of the cup containing the liquid and dissolved substances
when the bubbling has ceased.
f. Wash and rinse the cup with water.
g. Calculate the mass of carbon dioxide generated.
h. Calculate the mass of NaHCO3 reacted.
i. Calculate the percent by mass of the reacted NaHCO3 in the tablet.
2. Repeat the experiment with 5 mL vinegar + 30 mL water, 10 mL vinegar + 25 mL water, 15 mL
vinegar + 20 mL water, 20 mL vinegar + 15 mL water, 25 mL vinegar + 10 mL water, 30 mL
vinegar + 5 mL water, and 35 mL vingar instead of 35 mL of water.
3. Plot the “Calculated % by mass of the reacted NaHCO3 in a tablet” on the Y-axis versus the
“Volume of vinegar” on the X-axis.
Data Table:
Experiment #
1
2
3
4
5
6
7
8
Vinegar (mL)
0
5
10
15
20
25
30
35
water (mL)
35
30
25
20
15
10
5
0
Mass of cup + liquid (g)
Mass of tablet (g)
Mass of cup + all (g)
Mass loss (mass of CO2) (g)
Mass of NaHCO3 (g)
Calculated % by mass of
NaHCO3
Analysis Questions:
1. Write the chemical equation corresponding to the reaction that causes the bubbling when an
antacid tablet is dropped into water.
2. How do you determine the amount of carbon dioxide generated in the reaction?
3. How do you determine the amount of NaHCO3 consumed in the reaction?
4. How do you completely neutralize the NaHCO3 in the reaction?
5. How do you determine the percent by mass of NaHCO3 in an antacid tablet?
6. How do you interpret the graph obtained in terms of the concepts of limiting reactant?
7. How does the dissolving of CO2 in the reaction solution and the evaporation of water affect the
results of the analysis, respectively?
Conclusion:
What do the results indicate about the percentage of sodium hydrogen carbonate?
What do the results indicate about the limiting reagent?
What possible errors may have occurred that affected your results?
How could this experiment be improved to eliminate potential sources of error?