The Neutralizing Ability of Antacid Tablets Lab
Name____________________________________________________Date____________________Pd.____
Purpose: In this experiment you will measure the amount of stomach acid consumed (or neutralized)
by various antacid tablets. If you have a favorite one, bring a package to the lab (one color only).
Procedure: This experiment involves several steps. First the tablet is dissolved and an excess of
acid of known concentration is added to the tablet. The solution is then briefly heated to insure that
all of the antacid reacts. Finally, the remaining unreacted acid is titrated with base (NaOH) to
determine the amount left over and hence, the amount that reacted with the tablet. The chemical
reaction which occurs is:
NaOH + HCL  NaCL +H2O
A detailed description of the individual steps in the analysis follows.
Pre-lab questions:
1. What is the formula for hydrochloric acid? _________
2. When hydrochloric acid is added to water, it donates a hydrogen cation (H+) to a
water molecule, producing a hydronium cation (HO-) and an anion. Write this
reaction below:
3. A base is a substance that accepts a proton (H+) donated by an acid. In the
equation you wrote for question 2, water accepted the proton, so it is a base.
Antacids are bases. They are substances that are very good at accepting protons
(compared to water) and thus can reduce the amount of irritating acid (H+ or
H3O+) in your stomach. The hydroxide ion (OH-) is the most common base.
Carbonate ion (CO32-) is also a common base used in antacids. When an acid and
base react with each other, it is called a neutralization reaction. For example:
HBr (acid) + NaOH (base) ? H2O + NaBr (salt)
4. What type of reaction is this? ____________________________
5. For the following reactions (1) complete and balance the equation, (2) circle the
acid, (3) underline the base, and (4) double underline the salt.
a. ____LiOH + ____ HCl 
____H2O + ____ LiCl
b. ____HNO3 + ____Al(OH)3  ____H2O +____ Al( NO3)3
c. ____ H2SO4 + ____ KOH ____ H2O +____ K2(SO4)2
d. ____ CaCO3 + ____HClO4 ____ H2O +____ CaClO4 + ____ CO2
Procedure:
Indicator Investigation
1. Place 10 drops of 1 M HCl solution in a test tube, and place 10 drops of NaOH
solution in a different test tube.
2. Add 1 drop of phenolphthalein in each of the test tubes and swirl. If no color
change is present, add a second drop of phenolphthalein and swirl. Continue to
add only a single drop at a time (swirling after each drop) until a color change is
present. What do you observe? Record your observations.
3. Now, slowly drop HCl into the NaOH test tube one drop at a time. Gently stir the
solution to mix after each addition. Record your observations.
NOTE: Bromothymol blue is an indicator at a pH of 7
Preparation of Sample:
1. Weigh an antacid tablet using the electronic scale. Measure the mass of the tablet to the
nearest .1 gram. Then transfer the antacid tablet to a 250 ml Erlenmeyer flask. Record the
weight of the tablets you are going to analyze.
2. Using a graduated cylinder measure out 50.0 ml of hydrochloric acid solution. Add 50.0 ml of
hydrochloric acid solution (labeled 0.50 M) to the flask containing the tablet.
3. If the tablet does not dissolve readily, place the flask on a hotplate and boil gently until the
tablet has dissolved. Set it aside to cool. While it’s cooling, weigh out another
tablet of a different brand and proceed through steps 1 and 2.
The Titration:
1. Using a 100ml graduated cylinder measure out exactly 100 ml of NaOH (sodium hydroxide)
2. Add 10 drops of the indicator, phenolphthalein, to the cooled solution of antacid tablet
containing unreacted acid. The solution will be clear.
3. Read the initial volume of the graduated cylinder using the graduations on the graduated
cylinder and bottom of the meniscus. Record this as the initial volume for the first trial in your
data table.
4. Using a pipette add the sodium hydroxide to the flask. A spot of pink solution may appear of
the base solution needed to cause the solution in the beaker to change from clear to very pale
pink and stay pink for 15 seconds or more. This is harder than it may sound. The color
change should be observed with the addition of only one of two drops of the base solution and
you can’t go back if you add too much. Watch the solution in the flask. AS it gets more
difficult to get rid of the pinkish color when you swirl the flask, add smaller amounts.
Ideally, you should be adding it drop wise when you reach the point where color finally
changes from clear to pink (end point). Patience! The first time is the hardest. Once
you know how much it takes for a tablet, you can add slightly less in the next trial and add the
last milliliter of so drop wise and you won’t go past the end point.
5. Read and record the final volume. Calculate the total volume you used by subtracting the
initial from the final volume 100ml –final mL= amount used. Record this, too, in your data
table.
6. Repeat steps 1-5 for the second brand of antacid tablet.
Data Table 1 :
Indicator Investigation
Color of HCl +
Phenolphthalein
Color of NaOH +
Phenolphthalein
Color after adding
HCl to NaOH
Number of drops it took to turn
NaOH acidic
Data Table 2 :
Antacid Titration
Antacid
Mass (g)
Color before
titration
Color after
Titration
Initial
Volume
(mL)
Final
Volume
(mL)
Amount of
Amount of
NaOH used HCl
Neutralized
Brand #1
Brand #2
The Calculations:
The number of grams of stomach acid that is neutralized can be calculated using the expression:
grams stomach acid per tablet=10[25.0 – (ml base)(.50)]
If, for example, it took 8.0 ml of base to titrate the sample then,
grams stomach acid = 10[(25.0) – 8.0 (.50)] = 210 g
Record the value for each of your samples and the average value with your results
Data Table 3:
Class Data
Antacid Brand #1 Amount of HCl Neutralized
Antacid Brand # 2
Lab Group 1
Lab Group 1
Lab Group 2
Lab Group 2
Lab Group 3
Lab Group 3
Lab Group 4
Lab Group 4
Lab Group 5
Lab Group 5
Lab Group 6
Lab Group 6
Lab Group 7
Lab Group 7
Amount of HCl Neutralized
Wastes:
The HCl and NaOH used in this experiment are weak. They can be put into the sink with the water
running. The finished titrations are not hazardous and can also be rinsed down the drain.
Analysis Questions:
1. If you had a bad case of heart burn which antacid tablet would you use? Explain why you
would use this brand versus other brand? _________________________________________
__________________________________________________________________________
2. Explain how you recognized that the acid in the flask had been neutralized ( reached the
endpoint)? _________________________________________________________________
__________________________________________________________________________
3. List two other pH tests that you could use to determine the pH of the contents of the flask.
__________________________________________________________________________
__________________________________________________________________________
4. How would taking several antacid tablets affect your stomach? What outcome would you
expect, if you took too many antacid tablets, on your stomach?________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
5. How would increasing the amount of acid in your stomach ( eating spicy foods) affect the rate
at which an antacid tablet would work?___________________________________________
__________________________________________________________________________
__________________________________________________________________________
Lab Write up:
State the Problem:
________________________________________________________________________________
________________________________________________________________________________
Generate a hypothesis (Use If…Then…Because strategy)
________________________________________________________________________________
________________________________________________________________________________
Identify the Variables:
Independent Variable:_______________________________________________________________
Dependant Variable:________________________________________________________________
Conclusion:
Does your tablet neutralize 47 times its weight in stomach acid? You can answer this question for
your conclusion by dividing the mass of stomach acid by the mass of the individual tablet.