Determining The Amount Of Stomach Acid That Can Be Neutralized
By An Antacid Tablet
INTRODUCTION:
The active ingredients of antacid tablets are usually metal carbonates such as calcium carbonate or
hydroxides such as magnesium hydroxide. Stomach acid contains hydrochloric acid, HCl. When
hydrochloric acid reacts with calcium carbonate, the reaction is:
2 HCl(aq) + CaCO3(s)
®
CO2(g) + CaCl2(aq) + H2O(l)
.... (1)
When hydrochloric acid reacts with magnesium hydroxide, the reaction is:
2 HCl(aq) + Mg(OH)2(s)
® MgCl2(aq) + 2 H2O(l) .
.... (2)
The antacid you will be given may contain one or both, CaCO3 and/or Mg(OH)2 .
In this experiment, you will react an antacid tablet with HCl using a TITRATION procedure. (The TITRATION
in this experiment is a process in which a measured volume of HCl solution reacts with a known amount of
tablet until a desired END POINT is reached. The “end point” of a titration is the point at which an indicator
dye just changes colour. ) The amount of HCl needed in the reaction is used to see if your results agree with
the manufacturer ‘s claim of tablet composition and the amount of stomach acid the tablets can neutralize.
PRE-LAB QUESTIONS:
Note: most of these questions can be answered from reading the lab!
1. After eating an antacid tablet, most people tend to burp! Using Chemistry Principals, explain why.
2. In your own words, explain the basic purpose of a titration experiment.
3. What is the titrant in this experiment?
4. When properly cleaning a buret, why do you need to use some of the HCl solution to rinse out the water
droplets?
5. A mortar & pestle is an tool that has been used for many year to increase the surface area of reactants,
which term is used for the ‘bowl’ and which term is for the ‘grinding instrument’?
6. What does it mean to quantitatively transfer “something”? (quantitatively transferring a salt with dH2O)
7. Why is it essential to repeat a titration at least once?
MATERIALS
Apparatus:
(2) 150 or 250mL beaker
(1) funnel
(1) 250mL Erlenmeyer
Reagents:
100mL of 0.500M HCl
(1) Buret (50mL)
(1) mortar & pestle
(1) rubber stopper for buret
(1) scoopula
(1) stand & buret clamp
(1) dH2O wash bottle
(1) Antacid tablet
Distilled water
(1) dropper of methyl orange
PROCEDURE:
1. (a) Obtain 100 mL of 0.500 M HCl in a clean and dry 250 mL beaker.
(bi) *Proper cleaning of buret* An easy 3-step process: 1st) tap water – use as much as you want (no
need to dry); 2nd) dH2O (deionized water) – use sparingly (about 20—30mL) to remove ions left
by water; 3rd) your titrant – use sparingly (about 10-15mL) to remove remaining water droplets.
(bii) Obtain a burette, properly clean: (the following explains how to complete steps 2 and 3) close
the stopcock and pour in about 15mL of the dH2O (2nd) or HCl solution (3rd) using the funnel.
Stopper the burette, tilt the burette and roll the dH2O (or HCl solution) around inside the burette to
wash the inside of the glass tube. Remove the stopper, let some of the solution drain out through
the stopcock into waste beaker and the dispose of the remaining solution by inverting the burette
in a waste beaker (use a separate 250mL beaker for waste).
(c) Attach a burette clamp to a stand and secure the burette into the clamp. Fill the burette with HCl
to a point somewhat below the “zero” mark (using the funnel). Put the waste beaker below the
burette tip and drain a small portion of solution out to make sure the tip of the buret (below the
stopcock) is full. Record this initial volume of HCl in your data table. (note: the starting
volume does NOT have to be exactly at the zero mark… you just need an accurate reading!)
2. Ask your teacher for an antacid tablet (there will be several different ones, and your group will be
sharing data with the class) and find its mass using the electronic balance. Record this mass in
your data table and record the branding info of the tablet (ex. Tums® extra strength)
3. (a) Use a clean and dry mortar and pestle to carefully grind the tablet into a powder, making sure not
to lose any of the tablet.
(b) Obtain a 25mL graduated cylinder and fill with precisely 25mL of dH2O.
(c) Quantitatively transfer the powdered tablet into the 250mL Erlenmeyer flask using ALL of the
25mL of dH2O.
4. Add 4 or 5 drops of methyl orange indicator to the 250mL Erlenmeyer flask containing the antacid, put
the flask under the burette tip and adjust the height of the burette until the tip is about 5 cm above the
top of the flask. Place a piece of white paper under the flask to help make colour changes clear.
5. Add HCl from the burette at a moderate rate directly into the flask while swirling the beaker slowly.
Preferably, the stream of liquid from the burette should enter the liquid in the flask toward the side
rather than into the middle. As the added amount of HCl increases you should see a distinct PINK
colour in the solution where the stream of HCl enters. Eventually, the colour in the flask will resemble
a pink “streamer” that grows longer and longer. At this point, decrease the rate of flow from the
burette AND wash the sides of the flask with a small amount of dH2O. When the swirled solution
“blushes” pink (becomes pink for an instant), stop adding the HCl. Swirl for a second and then add
more HCl drop by drop with constant swirling (you may need to wash the sides of the flask
periodically). When the end–point pink colour forms and remains for 60 seconds, you are finished. (If
the end-point pink colour fades after a few seconds, add another drop until the colour persists.)
Record the “burette reading at the end of 1st titration”.
6. Re–load the burette with HCl solution and record the “burette reading at start of 2nd titration”.
(The burette does not have to be rinsed out with HCl; it already has HCl in it and can simply have
more HCl added.)
7. Rinse out the 250 mL flask (it doesn’t have to be dry) and repeat steps 2 to 5 a second time.
8. Re–load the burette with HCl solution and record the “burette reading at start of 3rd titration”.
9. Rinse out the 250 mL flask (it doesn’t have to be dry) and repeat steps 2 to 5 a third time.
10. Clean up. Make sure all glassware is rinsed with tap water. Any excess reagents are disposed of in
the waste beaker.
DATA AND OBSERVATIONS:
mass of 1st tablet = ___________
burette reading at the start of 1st titration = ___________
burette reading at the end of 1st titration = ___________
mass of 2nd tablet = ___________
burette reading at the start of 2nd titration = ___________
burette reading at the end of 2nd titration = ___________
mass of 3rd tablet = ___________
burette reading at the start of 3rd titration = ___________
burette reading at the end of 3rd titration = ___________
CALCULATIONS
1. Calculate the total volume of HCl added in each titration (it equals the difference between the starting
and ending volumes).
2. Use the total volume of HCl for each titration and the molarity of the HCl (0.500 M) to calculate the
total moles of HCl added in each of the three titrations. This is called the “experimental total moles of
HCl” for each tablet. Average the values for the “experimental total moles of HCl” and call the result
“average experimental total moles of HCl”. Note: If the values of “experimental total moles of HCl” for
two of the tablets are very close to each other and the third value is quite a bit different from the other
two, state that you are omitting the value that is too far from the others and average the remaining two.
3. The label on each package of your antacid will claim that each tablet contains ____mg of CaCO3 and
____mg of Mg(OH)2 . Use this information to calculate
a) the moles of CaCO3 in each tablet, and
b) the moles of Mg(OH)2 in each tablet.
4. Use equations (1) and (2) in the INTRODUCTION, and the results of Calculation 3, to calculate
a) the moles of HCl that react with the moles of CaCO3 in a tablet, and
b) the moles of HCl that react with the moles of Mg(OH)2 in a tablet, and
c) the total moles of HCl that are expected to react with both the moles of CaCO3 and moles of
Mg(OH)2 in each tablet. This is called the “expected total moles of HCl”.
5. Calculate the average percentage purity of the tablets from the following equation.
percentage purity =
x 100%
6. The CaCO3 and Mg(OH)2 referred to in Calculation 3 are called “active ingredients”. Calculate the total
mass of the active ingredients in a tablet. Express the total mass of the active ingredients in
“grams”. Using the “mass of 1st tablet” recorded in your Data Table, calculate the percentage of the 1st
tablet’s mass that was “active ingredients”. Repeat the calculation for each of the other tablets and
calculate the average of the percentages. As in Calculation 2, if the percentages for two of the tablets
are very close to each other and the third value is quite a bit different from the other two, state that you
are omitting the value that is too far from the others and average the remaining two.
7. a) Stomach acid has an average molarity of 0.010 M. Calculate the volume of stomach acid which
contains the “experimental total moles of HCl”, using the average molarity of stomach acid.
b) Stomach acid has a density of 1000 g/L. Calculate the mass of stomach acid, using the density
of stomach acid and the volume of stomach acid calculated in 7(a).
c) Some antacids claim they consume many times their own weight in stomach acid. Calculate the
average mass of the antacid tablets you used. As in Calculation 6, if the masses of two of the
tablets are very close to each other and the third value is quite a bit different from the other two, state
that you are omitting the value that is too far from the others and average the remaining two. How
many times larger than the average mass of the antacid tablets is the mass of stomach acid you
calculated in 7(b)?
CONCLUSION:
Discuss your hypothesis and whether you were correct or not.
Also, discuss sources of error in the experiment.