DO NOT WRITE ON THIS LAB!!!!
LAB # ________
Lab Instructions – My Tummy Hurts
Background: Your stomach is an amazing organ! The stomach begins the digestive process by
secreting a very strong/concentrated Hydrochloric Acid with a pH of about 2. The inside of your
stomach is normally protected from the strong HCl by a coating of mucous, but sometimes your
stomach produces too much acid (like with you eat too much spicy food, or you are very stressed)
that can eat through the mucous coating and actually start to digest your stomach lining. This hurts
and burns causing “heartburn”, stomach aches, and in the worst cases a sore or ulcer forms. A quick
relief for excess stomach acid is to tae an antacid. Antacids are really just bases (pH>7) that will
react with and neutralize the acid (pH<7) in your stomach to produce salt and water (pH = 7). With
the acid gone (or greatly reduced) the burning pain goes away and your stomach makes a new
protective mucous lining!
1. You will be trained to titrate acids and bases using a buret.
2. You will be trained to calculate how much stomach acid an antacid will neutralize.
3. You will be trained to determine which antacid is the best buy.
Materials:
safety goggles
1 buret, ring stand, and buret clamp
1 funnel – must fit in buret
1 – 250 ml flask
1 -100 ml graduated cylinder
1 mortar and pestle with plastic spoon
1 – 250 ml beaker to hold Na(OH)
balance
weighing paper
HCl
Na(OH)
Phenolphthalein with pipette
Procedure: Part 1 – Phenolphthalein
A. Phenolphthalein is an indicator that is commonly used in Chemistry. Like all indicator solutions,
it can be used to give a continuous pH reading of a liquid and will let you know when a pH
change has taken place by changing colors.
B. Your teacher will demonstrate the color changes of Phenolphthalein. Record the color of
Phenolphthalein in an acid solution and in a base solution in Table 1.
Part 2 – The Upset Stomach
A. You MUST keep your safety glasses on to protect you from the strong acids and bases used in
this lab.
B. Pick up a 250 ml Erlenmeyer flask. This will represent your stomach! Use a graduated cylinder
to carefully measure out 50 ml of Hydrochloric Acid (HCl) and pour it in the flask. You have
just eaten a large, spicy meal and your stomach is full of burning acid!
C. Choose an antacid you would like to test and record its name in Table 2.
D. Take a tablet of the chosen antacid and grind it into a powder using a mortar and pestle (this will
help the antacid tablet dissolve in your stomach acid).
E. Use a plastic spoon to scrape the powdered antacid tablet and weigh it (don’t forget to subtract
the weight of the paper you should be using when you weigh a powder). Record the weight of
the powdered tablet in Table 2. Clean and dry the mortar and pestle.
F. Pour all the powdered antacid tablet into your “upset stomach” flask of acid. Gently swirl to help
dissolve the antacid.
G. You can’t really see what happened in your “upset stomach” as you added the antacid but a
Neutralization Reaction quickly took place. Antacids are really just bases – like Mg(H)2. When
the antacid’s base enters your stomach it reacts with the stomach’s acid (HCl) to produce
harmless salt and water. There is now less acid in your stomach!
GOOD ANTACIDS CONTAIN MORE BASE AND WILLNEUTRALIZE MORE OF YOUR
STOMACH’S ACID!
H. How did your antacid do? Remember, you started with 50 ml of HCl in your “stomach”. How
much of this acid was neutralized by the antacid and how much acid remains in your “stomach”?
Part 3 will help you answer these questions!
Part 3 – Titrating With a Buret
A. Add THREE drops of Phenolphthalein Indicator Solution to your stomach flask and observe. Are
your stomach’s liquids still acidic (pH < 7) or did the antacid neutralize all the acid and turn your
stomach’s liquids slightly basic (pH > 7)?
B. The clear color in your “stomach” flask tells you the stomach liquid still has acid in it and the pH
is less than 7. The antacid did NOT neutralize the entire 50 ml of HCl stomach acid. You still
can’t tell if your antacid worked great (like the drawing below).
Or not so great (like the drawing below).
C. To find out exactly how well your antacid worked you are going to use a buret to add Na(OH)
base to you “stomach” flask. A buret is like a large, expensive pipette on a stand that measures
very accurately. The laboratory method you are going to use is called a titration. A titration is
when you find the unknown concentration of one liquid called a Titration. A titration is when
you find the unknown concentration of one liquid (the amount of HCl left in the “stomach”) by
adding another liquid (Na(OH) base) to it until a color change occurs.
D. Carefully move a buret to your table. Make sure the valve is in the OFF position so liquid will
not come out the end (see Sketch A). You should also make sure there is a funnel in the top of
your buret.
E. Carefully bring a beaker of Na(OH) base to your table. Use the funnel to help you carefully fill
the buret (from the top) with Na(OH) base, however, don’t try to fill it too fast or it will
overflow. You do not need to fill the buret to the 0 ml line. Anywhere close to, but below, the
0 ml line is fine. NEVER FILL THE BURET WITH THE STOMACH ACID/ANTACID
MIXTURE OR IT WILL CLOG THE BURET!
F. Notice the lines on the side of the buret. The numbered lines are milliliters (ml) and the lines in
between are tenths of a milliliter. These numbers tell you how many ml of liquid you have
released from the buret – NOT how many ml of liquid are in the buret. Also notice that the top
level of the Na(OH) in the buret tube dips down in the middle. When reading the volume of a
liquid you should read from the bottom of this curve (see Sketch A).
G. Place the beaker of Na(OH) beneath the tip of the buret. Make sure it is positioned to catch liquid
as it escapes. Turn the valve to the ON position (see Sketch A) and let a few ml of Na(OH)
escape from the tip of the buret into the Na(OH) beaker. This will remove any air from the buret
tip. Notice that you can adjust the valve so that Na(OH) come out of the tip in slow drips if
needed.
H. Turn the valve to the OFF position and move the beaker of Na(OH) out of your way. Place your
“stomach” flask beneath the tip of the buret. It’s time to titrate and find out how much HCl acid
remains in your “stomach”!
I. Look at the buret and determine the EXACT ml reading of Na(OH) in the tube (remember, you
do not need to begin at 0 ml). This number is your Initial Na(OH) Reading. Record this reading
in Table 2.
J. Add a small amount (1 – 2 ml) of Na(OH) from the buret to your “stomach” flask, then gently
swirl the flask. BE CAREFUL NOT TO BANG THE GLASS TIP OF THE BURET – IT
BREAKS! The 1 – 2 ml of Na(OH) you just added will neutralize 1 – 2 ml of the HCl remaining
in your “stomach” flask. Your goal is to neutralize ALL the remaining acid so you will know
how much was there. Remember, the phenolphthalein you added to the “stomach” flask will turn
pink the second all the acid is neutralized and the pH reaches 7. Continue to slowly add Na(OH)
to the “stomach” flask until it turn pink and stays a light pink color for at least 15 seconds. All
the acid is now neutralized!!
K. Look at the buret and determine the EXACT ml reading of Na(OH) in the tube. This number is
your Final Na(OH) Reading. Record this reading in Table 2.
L. Calculate the Total Na(OH) Volume you used to neutralize the remaining “stomach” acid by
using the following formula:
Final Na(OH) Reading – Initial Na(OH) Reading = Total Na(OH) Volume used
Example: Your Initial Na(OH) Reading was 3.2 ml and after neutralizing all the HCl your Final
Na(OH) Reading was 28.6 ml. The Total Na(OH) Volume used was 28.6 ml – 3.2 ml
= 25.4 ml
M. Pour the neutralized “stomach” acid down the sink and rinse out the “stomach” flask.
Part 4 – More Antacids!
A. Repeat the titration process using a SECOND antacid of your choice. Ad more Na(OH) to the
buret before beginning if needed, and don’t forget to add phenolphthalein to your “stomach”
flask! Record all data in Table 2.
B. Repeat the titration process using a THIRD antacid of your choice. Record all data in Table 2.
C. Look at the data you recorded in Table 2. You should now have a rough idea about which of the
antacids you tested neutralized the most stomach acid. After completing Part 5 you will know for
sure which of the antacids is the best!
D. When you have completed all your tests you should clean and return all your equipment. Your
supervisor will tell you what to do with the buret.
Part 5 – Calculating How Well Your Antacids Worked
A. Look at the pictures below to understand how titrating with Na(OH) helped you determine the
amount of stomach acid each antacid neutralized.
B. You began each experiment with 50 ml of hydrochloric acid (HCl) in your “stomach”. How
many ml of this acid did each antacid table neutralize? Calculate by using this formula:
ml of stomach acid
neutralized by the entire
antacid table
50 ml of acid
= to begin with
=
total Na(OH) volume used during
titration to neutralize the remaining
acid
Record this calculation for each antacid in Table 2.
C. It really isn’t fair to compare one antacid tablet to another because some tablet brands may work
better simply because they are much larger than the others. To make fair comparisons you should
calculate how much stomach acid can be neutralized by ONE GRAM of each antacid. Calculate
by using this formula:
ml of stomach acid neutralized
=
neutralized by 1 gram of antacid
ml of stomach acid neutralized by the entire tablet
weight (grams) of the powdered antacid tablet
D. Do you pay for what you get? Do expensive antacid tables neutralize more acid than cheaper
brands? To answer these questions you must calculate how much stomach acid can be
neutralized by a single penny’s worth of each antacid. Calculate by using this formula:
ml of stomach acid neutralized
by 1 gram of antacid
=
ml of stomach acid neutralized by the entire tablet
weight (grams) of the powdered antacid table
Your teacher will supply you with the cost of antacid tablets. Record your calculations in Table
2.