Stoichiometry: Determination of Percent by Mass of NaHCO3 in
Stoichiometric measurements are among the most important in chemistry, they indicate the
proportions by mass in which various substances react. Stoichiometry includes writing and
balancing chemical equations, stoichiometric coefficients, molar ratios of reactants and products,
limiting reagents, theoretical yields and percent yields.
Alka-Seltzer is an over the counter antacid and pain relief medication which is taken by
dissolving it in water before ingesting. This drug contains aspirin (acetylsalicylic acid), citric acid
and NaHCO3. Immediately after the tablet is placed in water, an acid-base reaction involving
sodium bicarbonate takes place which leads to the generation of carbon dioxide through bubbling.
HCO3 - (aq) + H+ (aq) → H2O (l) + CO2 (g)
The release of carbon dioxide results in a total loss in weight after the reaction. With the weight
loss, one should be able to calculate the amount of sodium bicarbonate that reacted, and determine
the percent by mass of NaHCO3 contained in Alka-Seltzer tablets.
The limiting reactant is the reactant that is completely consumed in a chemical reaction and limits
the amount of product.
Equipment, Chemicals and Supplies
6 Alka-Seltzer tablets
Vinegar (acetic acid ca. 5%), 150 mL
250 mL beaker
Graduated cylinder (50 ± 0.5 mL)
Electronic balance (± 0.01 g)
a. Wear goggles and lab coat throughout the experiment.
b. Do not eat or taste vinegar or Alka-Seltzer tablets supplied.
c. Chemicals/Solutions should be disposed of in the appropriate containers.
1. Measure 60 mL of deionized water using a graduated cylinder. Pour into a 250 mL beaker.
2. Weigh and record the total weight of the beaker with the water using an electronic balance.
3. Weigh and record the weight of an Alka-Seltzer tablet using a weigh boat.
4. Drop the tablet into the beaker, carefully swirl the beaker to ensure complete dissolution of the tablet.
5. Weigh and record the weight of the beaker and all the contents when the bubbling ceases. Refer to the data sheet.
6. Wash and rinse the beaker with water.
7. Calculate the mass of the carbon dioxide generated.
8. Calculate the mass of NaHCO3 reacted.
9. Calculate the percent by mass of the reacted NaHCO3 in the tablet.
10. Repeat the experiment with:
a. 10 mL of vinegar + 50 mL of water
b. 20 mL of vinegar + 40 mL of water
c. 30 mL of vinegar + 30 mL of water
d. 40 mL of vinegar + 20 mL of water
e. 50 mL of vinegar + 10 mL of water
Record values on the data sheet for each.
11. Plot the calculated percent by mass of the reacted NaHCO3 in the tablet versus the volume of vinegar used using the Excel spreadsheet provided on Canvas.
Report: A Template for the report is provided on Canvas. Be sure to follow the instructions in the
template for each section of the report.
Answer the following questions in the Discussion section of your report. You should consider
these questions as you are performing your experiment. Take enough notes so that you can answer
the questions after you have finished the experiment.
1. What is a limiting reactant? How does the graph change in correlation to the limiting reactant? Which would you identify as the limiting reactant in this experiment? (5 pts.)
2. What will be the effect on the reaction if HCl is used instead of vinegar? How will the graph look? (5 pts.)
3. Balance the chemical equation below; (5 pts.)
CO3 2- (aq) + H+ (aq) →CO2 (g) + H2O (l)
4. If water evaporates from the reaction beaker throughout the course of the experiment, how would this affect the percent mass calculation of NaHCO3? (6 pts.)
5. Predict what might happen if the Alka-Seltzer tablet was dissolved in 60 mL of vinegar. How will this affect the graph? (4 pts.)
Data Sheet Alka Seltzer Lab
Lab partner’s name
Experiment # Run 1 Run 2 Run 3 Run 4 Run 5 Run 6
Volume of vinegar
0 10 20 30 40 50
Volume of water
60 50 40 30 20 10
Weight of beaker
and liquid (g)
Weight of Alka-
Seltzer tablet (g)
Weight of beaker
with liquid +
weight of tablet (g)
Weight of beaker
with all contents
Weight loss (mass
Mass of NaHCO3
% by mass of the
NaHCO3 in a tablet
Show one sample calculation for how you calculated the mass of NaHCO3 from the amount
of CO2 released.