CHM 1025C/CHM 1032C                Name: ________________________   

 A Stoichiometric Analysis
     of Alka-Seltzer

Alka-Seltzer is one of the world's best known antacids.  Its main function is to absorb excess stomach acid.  The two ingredients that accomplish this are sodium bicarbonate (NaHCO3) and citric acid (C6H8O7).  Aspirin is also present in Alka-Seltzer tablets to reduce fever and relieve headaches,

In this lab, we are going to study the reaction that takes place between the NaHCO3 and C6H807

   NaHCO3(aq)   +   C6H8O7(aq)   ------>    H2O(l)  +   CO2(g) +   Na3C6H5O7(aq)

Whenever an Alka-Seltzer tablet is placed in water, the above UNBALANCED equation is what is taking place.  Note that without water, that tablet stays unreacted.  It is stored in a foil pouch to prevent water vapor in the air from causing the tablet to start reacting.  Na3C6H5O7 is called Sodium Citrate.


The purpose of this lab is to determine what mass of carbon dioxide is produced from the reaction of one tablet of Alka-Seltzer and water. 


Predict the mass of CO2 that will be produced by looking carefully at the ingredients of an Alka-Seltzer tablet.  You must balance the above equation and find out which reactant is the limiting reactant.  Using the amount of limiting reactant present, calculate the mass of CO2 that should be produced.   This is your hypothesis. 

Pre-Lab Calculations:

To predict the mass of CO2 produced when one tablet reacts in water complete the following in your Lab Notebook::

1.       Calculate moles of NaHCO3 in one unreacted tablet?  (You must look at the back of the package to get ingredients.  The ingredient masses are for one tablet.  Use the mass listed on the package to calculate moles or see Appendix A)

Calculated MOLES of NaHCO3  ___________________


2.      Calculate moles of C6H8O7 in one unreacted tablet?  (You must look at the back of the package to get ingredients.  The ingredient masses are for one tablet.  Use the mass listed on the package to calculate moles) 

Calculated MOLES of C6H8O7 _____________________

3.      There are several ways to determine which reactant limits a reaction.   The following will show you the steps of one method.  If your balanced equation above shows a 3 to 1 ratio between NaHCO3 and C6H8O7 respectively, you balanced the equation properly.  This is the ratio we will use to determine which reactant limits the reaction.  According to this ratio, if we do not have 3 times as much NaHCO3 as C6H8O7 then NaHCO3 limits the reaction.  If we have more than 3 times as much NaHCO3 as C6H8O7 then C6H8O7 limits the reaction.  So let’s do some calculations:

Fill in the moles of NaHCO3  calculated in step 1:

a.   ______moles of NaHCO3    ·         1 mole C6H8O7     =      _______moles C6H8O7                                                          3 moles NaHCO3

b.      From here you simply compare the moles of C6H8O7 calculated in step 3a (theoretical calculation) to the moles of C6H8O7  calculated in step two (actual moles).  If the moles of C6H8O7 in step 3a (theoretical calculation) are less than actual moles calculated in step 2 then NaHCO3 limits the reaction.   This means there is enough C6H8O7  to react all the NaHCO3 and we will have some C6H8O7  left over.  If the moles of C6H8O7 in step 3a (theoretical calculation) are more than actual moles calculated in step 2 then C6H8O7 limits the reaction because we do not have enough.   This means that C6H8O7 will run out before NaHCO3.

c.    What reactant limits the reaction   ________________

4.       Now you use the moles of the limiting reactant calculated in step 1 or 2 (depending on which one limits the reaction) and its ratio to CO2 to determine the amount of CO2 produced.   This remember is your hypothetical.

a.        ___moles of limiting reactant  ·  ratio of  moles CO2  to moles Limiting =

                                                                                       ___moles of CO2  produced


Hypothetical grams of CO2 produced is ____________



Record all measurements in your lab notebook on your data page. 

1. Add 35 ml of distilled water to a 250 mL beaker, cover it with a watch glass and weigh the beaker, water and watch glass on the top loading balance on your desk. 

2. Weigh one Alka-Seltzer tablet.

3. Add the masses from steps #1 & #2 to find the total mass

4. Add the tablet to the water and observe the reaction.  After the tablet has dissolved and bubbling has ceased, weigh the beaker and its contents.

5. Calculate the Mass of CO2 initially released (Step #3 – Step#4)

6.  Wait about 10 minutes and weigh the beaker again. 

7. New mass of CO2 released (Step #3 – Step#6)

8. Place the beaker on a hot plate set on LOW.  As the solution warms, observe if any more bubbles form.  (Be very careful not to overheat the beaker, or mass will be lost as water vapor.  If you see any condensation of water on the upper part of the beaker or the watch glass, remove it immediately from the hot plate and let it cool.) Weight the beaker and contents again.

9. Calculate the Total CO2 released. (Step #3 – Step#8)

10. From the Total CO2 released, calculate the amount of Sodium Bicarbonate that reacted. (Mass-Mass problem)

11.  Put about 10 ml of your solution in two separate test tubes, and add vinegar (acetic acid) to one tube and solid sodium bicarbonate to the other tube.  In which case did the reaction start again? 

12.  How much Sodium bicarbonate was left unreacted?

13.  Calculate the % unreacted sodium bicarbonate?




Data page:

1. Mass of beaker, watch glass and water:     __________ g

2. Mass of one Alka-Seltzer:                            _________ g

3. Total mass (beaker, watch glass, water and tablet):              ____________ g

4. Mass of beaker, watch glass and contents after reaction       ____________ g

5.  Initial Mass of CO2 Released:                                                ____________ g

6. Mass of beaker, Watch Glass, Contents ten minutes later     _____________g

7.  New Total Mass of CO2 Released after 10 minutes:              ____________ g  

8. Mass of beaker, Watch Glass, Contents after heating:          _____________g

9. Total Mass of CO2  released in the reaction (CO2 is the only gas):  ____________g

10. Mass of Sodium Bicarbonate that reacted (mass-Mass Problem-Show Work):

__Step#9_Grams CO2_= ____?_____Grams NaHCO3



11. Which Test tube began reacting again?    Vinegar    or  Sodium Bicarbonate


12. Mass of Sodium Bicarbonate unreacted? (1.00 g – Step#10)  ___________g


13. Per Cent Sodium Bicarbonate unreacted:                                 ___________%





Post Lab Report:

1. What is the limiting reactant in an Alka-Seltzer tablet? 





2. Why is there unreacted sodium bicarbonate remaining after you dissolve the Alka Seltzer tablet?