(del) Open Source CA Textbook - Chemistry

OLD Unit 4: Gases and Their Properties

Unit 2: Chemical Bonding

OLD Unit 5: Acids and Bases

Antacid Lab


Indigestion and heartburn are the symptoms resulting when the amount of hydrogen ions exceeds a certain concentration (i.e.: you have too much acid in your stomach).  One method to relieve these symptoms is to neutralize the excess acid with a non-toxic chemical reagent that maintains the contents of the stomach at an appropriate concentration of acid.

Rolaids, a commercial antacid tablet, claims to neutralize 47 times its weight in excess acid (i.e. if you ate 1.0 grams of Rolaids, you would neutralize 47 grams of stomach acid).  In this experiment you will attempt to determine the validity of this claim, as well as to determine which is the best antacid.  You will add an excess of 0.14 M HCl acid (approximately the concentration of stomach acid) to a known amount of antacid tablet.  The antacid tablet should neutralize some of this acid.  You will then titrate the whole solution with a base of known concentration (0.14M NaOH) to determine the amount of HCl not neutralized by the antacid.  This process is called “back titration”.  From this information you can calculate the weight of the acid neutralized by the antacid tablet.  Before beginning to collect data, read through the procedure below and decide what data you need to collect.


1)     Grind one antacid tablet with a mortar and pestle.  After grinding, place about 1.00g of the powder into a 250mL Erlenmeyer flask.  Be sure to record the exact mass of the ground tablets. Although you should have around 1.00g, having exactly that amount is not important.

2)     Using a graduated cylinder, add approximately 75.0mL of the 0.14M HCl into the flask containing the antacid. This acid has about the same molarity as the acid in your stomach. Accurately record the volume of HCl used. You should have 3 significant figures in your measurement.

3)     Add 3 drops of phenolphthalein indicator to the acid / tablet mixture. Gently swirl the solution until the contents are well mixed and most of the tablet has dissolved.

4)     At this point in the experiment, most of the stomach acid has been neutralized by the tablet; however, the pH of the solution is still acidic as seen by the phenolphthalein indicator not having turned the solution bright pink. To find out how much acid remains, we will titrate the solution with 0.14M NaOH using a burette.  Record the starting volume of the NaOH in the burette so that you can determine how much total base you had to add to the solution.

5)     Place the stomach acid solution under the burette and open the stopcock so that NaOH is added drop-wise. Swirl occasionally. You will want to continue adding drops of NaOH until you reach the endpoint of the titration. You reach this point when the solution turns pink for a sustained period of about 10 seconds and then shifts back to clear. Record the final volume of base used in the data table. The amount of base used equals the amount of acid that was not neutralized by the antacid tablet.

DATA TABLE (if you do more than 2 tablets, record data in your lab notebook)


Antacid Name:

Antacid Name:

Mass of Antacid (g)



Starting Volume of 0.14M HCl in Erlenmeyer Flask (mL)



Starting Volume of NaOH in Burette (mL)



Ending Volume of NaOH in Burette (mL)



D Volume of NaOH in Burette (mL)



Volume of Acid Neutralized by Antacid (mL). Subtract DVolume from starting volume of HCl.



Mass of HCl neutralized by antacid (1mL HCl = 1g HCl). Put the number from above, but switch the units from mL to g.



Effectiveness of Antacid (grams of acid neutralized / grams of antacid) This ratio allows us to compare all the antacids regardless of how much tablet we started with.




·       Is this a neutralization reaction? How do you know?

·       Assuming the calcium carbonate is the only substance in the antacid tablets to react with hydrochloric acid (HCl), write a balanced equation for this reaction.

·       Which is the best antacid? How do you know?

·       Do you think that this activity really demonstrates which brand of antacid works best?  How confident are you in this?

·       How might the results of this experiment influence your decision to purchase antacids in the future?


·       IMPRESS MADDIN: Go to HEB and find the price and mass for each of the brands of antacid. (For example, TUMS might cost $3.50 for 20oz of antacid). Using this information, figure out which antacid is actually the best buy in terms of number of grams of acid neutralized per dollar.


Data Table for Antacid Lab 


Antacid =

Antacid =

Antacid =

Mass of Antacid





Volume of HCl





Volume of NaOH





Volume of Acid Neutralized by NaOH




Volume of Acid Neutralized by Antacid




Mass of HCl neutralized by antacid (1mL=1g)




Ratio= (mass of HCl neutralized by antacid) / (mass of the tablet)






  Effectiveness of Antacid (How much Stomach Acid each brand neutralized)
Trial # Rollaids Extra Strength Rollaids Regular Strength TUMS Regular Strength HEB Brand
1 61.3 46.8 45.7 38.3
2 52.5 54.2 53.0 45.5
3 55.7 39.8 54.0 48.9
4 46.2 50.1 24.7 54.1
5 45.3 46.3 53.2 42.8
6 58.4 41.1 46.0  



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