Lab: Alka Seltzer
Introduction:
The extent of a chemical reaction is determined by the amount of reacting materials, or reactants,
present in the reacting system. Reagents are generally not mixed in exact amounts needed for the reaction to
take place; there is usually one of the reacting materials added in excess. The excess reagent, usually a noncritical reagent or an inexpensive material, is used to insure that the entire critical reagent is used up so that
there will be a maximum amount of product formed. Also, an excess of one reactant can cause the overall
reaction to take place at a faster rate. Once the critical reactant is used up, the reaction will stop. That critical
reactant is called the limiting reagent, that is, it is the reactant in the chemical reaction that is used up first.
Alka Seltzer, and similar over-the-counter combination antacid and pain relief medications, contain
aspirin (acetylsalicylic acid), citric acid, and NaHCO3 as the active ingredients. This medication is designed to
be taken by dissolving it in water before ingesting. The carbon dioxide, from the reaction, escapes into the
atmosphere and results in a weight loss from the total mass of the reactants. Using the loss in mass, one can
calculate the amount of sodium bicarbonate reacted, and determine the percent by mass of NaHCO3 contained
in Alka Seltzer tablets.
In this experiment, the Alka Seltzer tablets will be reacted with differing amounts of acetic acid,
HC2H3O2, to produce the carbon dioxide. From the data, and graph of the data, the mass of the sodium
bicarbonate in the Alka Seltzer tablets can be calculated, and the identity of the limiting reagent can be
determined.
Materials: – per lab bench
2 tablets, Alka Seltzer Tablets
Acetic acid - 4.5 to 5% (vinegar)
2 - 250 mL beakers
10 mL graduated cylinder
50 mL graduated cylinder
Glass stirring rod
Pre-lab Questions:
1. Sodium bicarbonate reacts with acetic acid to produce sodium acetate, carbon
dioxide, and water. Write the complete balanced equation below.
2. What is the mole ratio between all compounds of this reaction?
3. Can you tell by the balanced equation what the limiting reactant is? Why?
4. For the experiment the total volume of the liquid must equal 40.0 mL for each trial. Fill in the following
data table to indicate the amount of water to be added to the vinegar.
Volume of Vinegar (mL)
0.0
5.0
10.0
25.0
30.0
Volume of water (mL)
5. Use the following data to answer the questions…
Vinegar
Water
Mass
Mass Alka
Mass
Mass of
Mass of
(mL)
(mL)
liquid in
Seltzer
beaker &
beaker &
CO2 (loss
beaker (g)
Tablets (g)
contents
contents
of mass)
before rxn.
after rxn.
(g)
(g)
(g)
A
B
A+B
C
(A + B) –
C
5.0
35.0
145.0
3.23
20.0
20.0
148.0
3.27
148.23
147.60
0.63
150.12
a. Fill in the missing data from the second trial.
b. To determine the mass of NaHCO3 in the Alka Seltzer tablet, you must use a stoichiometric calculation
in which you start with the mass of CO2 produced and determine the amount of NaHCO3 that must
have reacted to produce that. Find the mass of NaHCO3 in each tablet given in the data table above.
(Hint: Stoichiometry: Mass of CO2 to Mass of NaHCO3)
Trial 1:
Trial 2:
6. The % NaHCO3 in each Alka Seltzer tablet can be determined by dividing the grams of NaHCO3in
the tablet (your answers from 5b) by the total mass of the tablet (from the data table). Find the %
NaHCO3 for each tablet in the data table above.
Trial 1:
Trial 2:
Procedure:
1. Record the amount of vinegar you and your partner will be testing here: _____mL &____mL and in data
Table 1.
2. Measure the first amount of vinegar using a graduated cylinder and pour into a 250 mL beaker.
3. Measure the amount of water you would need to add to make the total liquid volume = 40.0 mL using a
graduated cylinder and pour into the same 250 mL beaker; stir.
4. Record the mL of water in data table 1.
5. Mass the beaker w/liquid and record in data table 1.
6. Mass an Alka Seltzer tablet and record in data table 1.
7. Place the Alka Seltzer tablet into the 250 mL beaker w/water.
8. When the bubbling has stopped, stir the solution to make sure the tablet has completely dissolved and
to remove any visible bubbles of carbon dioxide. Gently tap the stirring rod against the inside of the
beaker to remove any drops of liquid.
9. Mass the beaker and its contents. Record the mass and record in data table 1.
10. Dispose of the solution in the beaker down the sink and wash the glassware!
Data: Table 1: Partner Data
Exp. Volume
of
#
Vinegar
(mL)
Volume
of
Water
(mL)
A
Mass of
beaker
& liquid
(g)
B
Mass of
Alka
Seltzer
tablet
(g)
A+B
C
Total mass
of beaker
and
contents
before rxn.
(g)
Total
mass of
beaker
and
contents
after
rxn. (g)
(A + B) - C
Mass of
CO2 lost
(g)
1
2
Calculations: Show ALL of your calculations used to complete Data Table 1.
Trial 1:
Trial 2:
Mass of
NaHCO3 in
tablet (g)
%
NaHCO3
in tablet
((mass
NaHCO3/
mass
tablet) x
100)
Data: Table 2: Class Data
Volume
of
Vinegar
(mL)
0.00
5.00
10.00
15.00
20.00
25.00
25.00
30.00
Volume
of
Water
(mL)
40.00
35.00
30.00
25.00
20.00
15.00
15.00
10.00
Mass of
beaker
& liquid
(g)
197.9
147.04
194.43
196.6
197.95
145.07
147.42
147.21
Mass of
Alka
Seltzer
tablet
(g)
1.68
1.36
1.35
3.20
2.02
2.13
1.65
1.83
Total
Total
mass of mass of
beaker
beaker
and
and
contents contents
before after rxn
rxn (g)
(g)
199.29
148.10
195.41
197.75
99.32
146.24
148.65
148.50
Mass
of CO2
lost
0.29
0.30
0.37
0.52
0.63
0.62
0.48
0.55
Mass of
NaHCO3 % NaHCO3 in
in
tablet (mass
tablet NaHCO3/mass
(g)
tablet) X 100
Analysis Questions:
1. Why must the mixture in the beaker be stirred after the reaction is completed?
2. Which substance (Alka-Seltzer OR Vinegar) do you think was used up in the reaction? Explain why.
3. Explain TWO sources of error that could have occurred in this lab and how each would affect the
results.
4. On the back, create a bar graph using the % of reacted NaHCO3 (Y-axis) versus the volume of vinegar
used X-axis) to determine if you were correct in your identification of the limiting reactant in this
reaction. Be sure to include a title, key and label your X & Y AXIS.
5. Look at the class data for % NaHCO3 vs. mL vinegar used. What trend do you notice?