Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
LAB 13
ANTACIDS
Introduction:
Chemists are often rather puzzled about how to check the claims that manufacturers
make about their products. For example, there are no reproducible experimental techniques
available for determining how fast a particular product cures a headache, relieves an upset
stomach, or revives tired blood. The scientific approach is to try to duplicate as many
conditions as possible and control the remaining variables one at a time.
Next to treating pain, one of the most frequent uses of over-the-counter medicines is
the treatment of indigestion caused by an overproduction of stomach acid (HCl). The number
of over-the-counter antacids is almost overwhelming, although the variation of ingredients is
small. The most common antacids contain one or more of the following: sodium bicarbonate,
calcium carbonate, magnesium carbonate, aluminum hydroxide, magnesium hydroxide, and
magnesium trisilicate. Each of these compounds can neutralize stomach acid.
Table I – Antacid Reactions
Basic Ion
Compound
bicarbonate
hydroxide
carbonate
Reaction
NaHCO3 or KHCO3
HCO3- + HCl ↔ Cl- + H2CO3*
Mg(OH)2, Al(OH)3,
or NaAl(OH)2CO3
CaCO3, MgCO3,
or NaAl(OH)2CO3
OH- + HCl ↔ Cl- + H2O
CO32- + 2 HCl ↔ 2 Cl- + H2CO3*
trisilicate
Mg2Si3O8
Si3O84- + 4 HCl ↔ 4 Cl- + 3 SiO2 + 2 H2O
citrate
Na3C6H5O7
C6H5O73- + 3 HCl ↔ 3Cl- + H3C6H5O7
tartrate
Na2C4H4O6
C4H4O62- + 2 HCl ↔ 2 Cl- + H2C4H4O6
*carbonic acid is unstable and decomposes into water and carbon dioxide in solution
The various antacids have advantages and disadvantages. Sodium salts should be
avoided by individuals on low-sodium diets. Magnesium salts may have constipating effects.
Combinations of aluminum and magnesium salts are sometimes formulated in an attempt to
prevent side effects.
Effervescent antacids utilize citric acid and sodium bicarbonate in a reaction to produce
carbon dioxide bubbles. More than 85% of the bicarbonate of Alka-Seltzer® is consumed by the
effervescent action. Do you think that it is possible to determine the acid neutralizing capability
of an antacid by simply measuring the amount of a single chemical contained in the antacid?
* carbonic acid (H2CO3) is unstable and decomposes in solution, forming water and carbon dioxide
Page 1 of 7
2011
Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
LAB 13
Many questions arise when considering the claim that a particular antacid “consumes 47
times its own weight in excess stomach acid.” What does “consumes” mean? What is stomach
acid? What is “excess stomach acid”? Do antacids have side effects? What does it mean that
an antacid “works well”? One of your tasks is to come with a question regarding antacids that
we will be able to answer by utilizing acid-base analysis techniques.
Table II – Major Commercial Antacid Products
Product Name
Alka-Seltzer® Blue
Alka-Seltzer® Gold
Bromo-Seltzer®
Brioschi®
Bisodol®
Phillips’ Milk of Magnesia®
Tums®
Alka 2®
Titrilac®
Amitone®
Maalox®
Mylanta®
Creamalin®
Di-Gel®
Camalox®
Bisodol® Low-Sodium
Rolaids®
Gelusil®
Eno®
Antacid Ingredients
NaHCO3 + citric acid†
NaHCO3 + citric acid
NaHCO3 + citric acid‡
NaHCO3 + citric acid
NaHCO3 + MgCO3
Mg(OH)2
CaCO3
CaCO3
CaCO3
CaCO3
Mg(OH)2 + Al(OH)3
Mg(OH)2 + Al(OH)3§
Mg(OH)2 + Al(OH)3
Mg(OH)2 + Al(OH)3§
Mg(OH)2 + Al(OH)3 + CaCO3
Mg(OH)2 + CaCO3
Mg(OH)2 + CaCO3
Mg2Si3O8 + Al(OH)3
Na2C4H4O6 + Na3C6H5O7
Equipment and Supplies:
1 M hydrochloric acid
1 M sodium hydroxide
potassium hydrogen phthalate
phenophthalein
commercial antacids
burets
400-mL beakers
250-mL flasks
mortar & pestle
weighing boat
†
contains acetylsalicylic acid (Aspirin®)
contains acetaminophen (Tylenol®)
§
contains a silicone defoaming agent
‡
Page 2 of 7
2011
Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
LAB 13
Procedure:
Part 1 – Preparation of “Synthetic Stomach Acid”
You will be provided with a starting solution of hydrochloric acid. Stomach acid is
approximately one-tenth as concentrated as your starting acid. To synthesize the stomach acid,
you will dilute the acid and then measure its concentration. In order to determine the
concentration of your acid and base, you will titrate them with a measurable standard;
potassium hydrogen phthalate (KHP).
Before starting, label one of your burets “acid” and the other “base.” Always use them with the
solution for which they are labeled. Do not mix the acid and base solutions in the buret.
1. Fill a 250-mL flask about one-fourth with distilled water. Place about 25 mL of the
starting 1 M HCl solution in the flask. Record the volume of starting 1 M acid used.
Dilute the solution with distilled water to about 250 mL. Use a clean, dry graduated
cylinder and record the total volume to the nearest 0.1 mL, transferring the acid into a
clean, labeled 400-mL beaker.
2. Place about 25 mL of the starting 1 M NaOH solution in another 250-mL flask. Record
the volume of starting 1 M base used. Dilute the solution with distilled water to about
250 mL. Use a clean, dry graduated cylinder and record the total volume to the nearest
0.1 mL, transferring the base into a clean, labeled 400-mL beaker.
You have now prepared the two stock solutions you will use for the antacid tests. Before the
solutions can be used, their concentrations must be known. You will determine the
concentration of the base solution first.
3. Clean and rinse your “base” buret. Then, rinse the buret with about 10 mL of your
diluted base solution. Fill the buret and record the initial base volume to the nearest
0.01 mL.
4. Using an analytical balance (±0.0001 g), mass out between 0.7 and 0.9 gram of the KHP
(also called potassium biphthalate, KHC8H4O4) solid and record this mass on your data
sheet. Place the solid into your empty flask and add approximately 50 mL of distilled
water. Stir the solution until all of the solid dissolves. Add three drops of
phenophthalein indicator to the solution.
5. Titrate the KHP solution with your base to the pink end-point. Record the final volume
of the base solution in the buret and calculate the volume of base used in the titration.
“All your base are belong to KHP!”
6. Repeat steps 4 & 5 again to check your measurement.
Page 3 of 7
2011
Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
LAB 13
Having standardized your base solution, you can now determine the concentration of the
synthetic stomach acid.
7. Clean and rinse your “acid” buret. Then, rinse the buret with about 10 mL of your
diluted acid solution. Fill the buret and record the initial acid volume to the nearest 0.01
mL.
8. Add about 20 mL of your acid solution to a clean, dry 250-mL flask and record the final
acid volume on the buret. Add between 50 and 75 mL of distilled water to the flask,
along with three drops of phenophthalein indicator.
9. Titrate the acid solution with your base to the pink end-point. Record the final volume
of the base solution in the buret and calculate the volume of base used in the titration.
“All your base are belong to acid!”
10. Repeat steps 8 & 9 again to check your measurement.
Before you do the antacid analysis part of the experiment, you should verify
that your acid and base concentrations are within the desired range.
Part 2 – Antacid Analysis
In this part of the experiment, you will use the acid and base solutions that you prepared to
determine how much excess stomach acid will be neutralized by an antacid.
1. Record the mass of one dose of an antacid (read the label to determine dosage). If
using an antacid tablet, crush the tablet using a clean mortar and pestle.
2. Weigh precisely about 0.2 g of the antacid using a clean, dry weighing boat. Transfer all
of this sample into a clean, dry 250-mL flask. Add between 19-21 mL of your stomach
acid to the flask with stirring to dissolve the antacid. Record initial and final buret
readings to the nearest 0.01 mL. Calculate the volume of acid added to the flask.
3. Add three drops of phenolphthalein indicator to the flask and titrate with your base to
the pink end-point. Record the initial and final volumes of the buret and calculate the
volume of base used in the titration.
4. Repeat steps 1 – 3 for two other antacids.
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Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
LAB 13
ANTACIDS DATA SHEET
Lab Partner:
_________________________
Part 1 – Preparation of “Synthetic Stomach Acid”
Dilution of Starting Acid
Volume of starting acid (mL)
__________
Total volume after dilution (mL)
__________
Dilution of Starting Base
Volume of starting base (mL)
Total volume after dilution (mL)
__________
__________
Determination of the Concentration of Standard Base
Mass of KHP (g)
__________
__________
Initial base volume (mL)
__________
__________
Final base volume (mL)
__________
__________
Volume of base used (mL)
__________
__________
1. Calculation of standard base NaOH concentration (show work for one trial):
Concentration of NaOH (M)
__________
__________
Average concentration of NaOH in standard base (M)
Determination of the Concentration of Synthetic Stomach Acid
Initial acid volume (mL)
__________
Final acid volume (mL)
__________
Volume of acid used (mL)
__________
__________
__________
__________
__________
Initial base volume (mL)
__________
__________
Final base volume (mL)
__________
__________
Volume of base used (mL)
__________
__________
2. Calculation of synthetic stomach acid HCl concentration (show work for one trial):
Concentration of HCl (M)
__________
__________
Average concentration of HCl in stomach acid (M)
__________
Page 5 of 7
2011
Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
Part 2 – Antacid Analysis
Name of antacid
Mass of antacid – 1 dose (g)
Mass of antacid sample (g)
LAB 13
__________
__________
__________
__________
__________
__________
__________
__________
__________
Initial acid volume (mL)
__________ __________ __________
Final acid volume (mL)
__________ __________ __________
Volume of acid used (mL)
__________ __________ __________
3. Calculation of moles of stomach acid used (show work for one trial):
Moles of HCl used (mol)
__________
__________
__________
Initial base volume (mL)
__________ __________ __________
Final base volume (mL)
__________ __________ __________
Volume of base used (mL)
__________ __________ __________
4. Calculate of moles of standard base used to titrate excess acid (show work for one trial):
Moles of NaOH required to neutralize excess acid (mol)
__________ __________ __________
5. Calculation of moles of stomach acid neutralized by the antacid (show work for one trial):
Moles of stomach acid neutralized by the antacid (mol)
__________ __________ __________
6. Calculation of moles of stomach acid that would be neutralized by one dose of the antacid
(show work for one trial):
Moles of stomach acid neutralized by one dose of the antacid (mol)
__________ __________ __________
Page 6 of 7
2011
Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 14 – Acids and Bases
LAB 13
Name of antacid
__________ __________ __________
7. Calculation of grams of acid neutralized by one dose of antacid (show work for one trial):
Grams of acid neutralized by one does of an antacid (g)
__________ __________ __________
8. Calculation of the relative efficiency of the antacid (show work for one trial):
mass of acid neutralized
relative ef
iciency mass of one dose of antacid
Relative efficiency of the antacid: __________ __________ __________
9. Calculation of the cost efficiency of the antacid (show work for one trial):
Note: this calculation may require some research at your favorite pharmacy
cost ef
iciency relative ef
iciency Cost efficiency (g/¢)
grams of one dose of antacid
cent
__________
__________
__________
Formula of the active ingredient(s) in the antacid
__________
__________
__________
__________
__________
__________
Molar ratio of HCl to active antacid ingredient (see Tables I & II in introduction)
__________ __________ __________
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