Project #55004 - Limiting Reagent and Determination of NaHCO3 in Alka Seltzer Tablets



Limiting Reagent and Determination of NaHCO3 in Alka Seltzer Tablets

A formal lab report is REQUIRED for submission of this lab. Please see How to Write a Formal Lab in Chemistry in Blackboard.


The extent of a chemical reaction is determined by the amount of reacting materials, or reactants, present in the reacting system. Reagents are generally not mixed in exact amounts needed for the reaction to take place; usually one of the reacting materials is in excess. The excess reagent, usually a non-critical reagent or an inexpensive material, is used to insure that the limiting reagent is used up so that there will be a maximum amount of product formed. Also, an excess of one reactant can cause the overall reaction to take place at a faster rate. Once the limiting reactant is used up, the reaction will stop.

Alka Seltzer contains aspirin (acetylsalicylic acid), citric acid, and NaHCO3 as the active ingredients. This medication is designed to be taken by dissolving it in water before ingesting. When the tablet is placed in water, an acid-base reaction involving sodium bicarbonate and the citric acid takes place resulting in the generation of carbon dioxide which is visible by active bubbling of the mixture. The carbon dioxide, from the reaction, escapes into the atmosphere and results in a weight loss from the total mass of the reactants. Using the loss in mass, one can calculate the amount of sodium bicarbonate reacted, and determine the percent by mass of NaHCO3 contained in Alka Seltzer tablets.

The equation for the reaction is:


NaHCO3 + CH3OOH à CO2 + H2O + NaCH3COO


In this experiment, the Alka Seltzer tablets will be reacted with differing amounts of acetic acid (CH3OOH) to produce carbon dioxide. From this data the average mass of sodium bicarbonate in an Alka Seltzer tablet can be calculated and the identity of the limiting reagent can be determined.


The chemicals used in this experiment do not pose any hazards. Vinegar ( 5% acetic acid) is not harmful. However, in the event of skin contact, wash the affected area with water. Do not drink any of the Alka Seltzer solutions or use any of the laboratory supplied tablets for medication. Even if the tablets are sealed in foil wrappers, they cannot be assumed to be free of laboratory chemical contamination. Dispose of all materials in a sink.


  • 8 tablets, Alka Seltzer Tablets (Bayer Corporation) or generic equivalent

  • Acetic acid, 4.5 to 5% (vinegar)

  • 2 250 mL beakers 10 mL graduated cylinder

  • 50 mL graduated cylinder

  • glass stirring rod


  1. Measure 35 mL of water into a 250 mL beaker. Weigh the beaker and water and record the mass on Table 1 (provided).

  2. Weigh and record on the table provided the mass of an Alka Seltzer tablet.

  3. Place the Alka Seltzer tablet into the water in the 250 mL beaker.

  4. When the bubbling has ceased, stir the solution to ensure complete dissolution of the tablet and to remove any visible bubbles of carbon dioxide. Gently tap the stirring rod against the inside of the beaker to remove any drops of liquid before weighing the beaker and its contents.

  5. Weigh the beaker and its contents. Record the mass.

  6. Dispose of the solution in the beaker as instructed.

  7. Repeat the experiment using 5 mL of acetic acid (vinegar) and 30 mL of water in the 250 mL beaker. Remember to weigh each Alka Seltzer tablet individually for each trial. NOTE: once the reaction has started, you may use a second 250-mL beaker to run the next trial in the experiment. You can continue to alternate beakers, running the additional experiments in simultaneous, but staggered trials. Be sure to keep track of the mass of each beaker used, as the exact mass of each beaker is different.

  8. Repeat the experiment a third time using 10 mL acetic acid (vinegar) and 25 mL of water in the 250 mL beaker.

  9. Continue to repeat the experiment 5 more times increasing the volume of the acetic acid by 5 mL and decreasing the volume of water by 5 mL for each new trial as indicated in Table 1.



Calculations and Results


  • Calculate the mass of carbon dioxide generated for each of the trials. Show at least one fully worked example of this calculation in your final lab report

  • Calculate the mass of NaHCO3 reacted for each of the trials. Show at least one fully worked example of this calculation in your final lab report.

  • Calculate the percent by mass of NaHCO3 in each tablet of Alka Seltzer . Show at least one fully worked example of this calculation in your final lab report.

The formula is: (mass of NaHCO3 in tablet/ total mass of tablet) x 100%

  • Plot a graph of the percent by mass of the reacted NaHCO3 in each tablet versus the volume of vinegar used. Refer to your Guide to Graphing if you forget how to do this properly.

Data and Results



Run Number

Volume of Acetic Acid


Volume of Water


Mass of Beaker and Liquid


Mass of Alka Seltzer Tablet


Total mass of Beaker and Contents Before Reaction


Total mass of Beaker and Contents After Reaction



Mass of CO2



Mass of NaHCO3

in Tablet



% of NaHCO3

in Tablet





























































































  1. This reaction only requires 35 mL of liquid. Why is the reaction run in a 250 mL beaker?

  2. The Alka Seltzer tablet contains citric acid which causes it to react in pure water. Why is acetic acid added to the reaction?

  3. Why must the mixture in the beaker be stirred after reaction is completed?

  4. With reference to your graph of reacted NaHCO3 versus the volume of vinegar… Which species of chemical is the limiting reactant in this reaction? Explain how you know this.

  5. Suggest reasons why the percent of NaHCO3 may vary in the different Alka Seltzer tablets.

  6. Carbon dioxide dissolves in water and water evaporates into the atmosphere during the experiment. Discuss how these two factors affect the results of your analysis in this experiment.











Subject Science
Due By (Pacific Time) 01/31/2015 01:00 pm
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