Alka-SeltzerTM Analysis
Pre-Lab:
Watch the video that accompanies this week’s lab experiment and then
answer these questions in your lab notebook (so that you make a copy
of them) and submit the white copy at the start of lab:
1. What is the volume of a sample of carbon dioxide gas collected in a
buret at 21.2 ˚C and 752 torr if it has a mass of 58.4 mg?
2. A sample of gas is collected in a buret. It has a volume of 22.7 mL
when the pressure is 771 torr. If the volume is reduced to 18.5 mL
at a constant temperature, what is the new pressure?
3. Calculate the molar mass of a 0.242-g sample of gas known to
occupy 93.0 mL at 22.3 ˚C and 676 torr.
Objective:
Determine the sodium hydrogen carbonate content of an Alka-Seltzer™
tablet. In the process, gain familiarity with the behavior of gases and
calculations involving gas mixtures and reactions that have a gaseous
product.
Method:
The Alka-Seltzer™ tablet contains a mixture of ingredients, but the
hydrogen carbonate ion content is what causes the neutralization of
acid in the stomach via the reaction:
NaHCO3(aq) + HCl(aq) → H2O(l) + NaCl(aq) + CO2(g)
As it turns out, there is some citric acid already in the tablet that will
contribute to the reactant side of this equation when the tablet is
dissolved. To ensure that all of the hydrogen carbonate ions are
reacted, you will dissolve the tablet in an excess HCl solution. By
measuring the amount of gas that is evolved, you can determine the
amount of hydrogen carbonate ion present, since for every one mole
reacted, one mole of carbon dioxide gas is evolved.
Procedure:
1. Pour 500 mL of water into a cut-off 2-L soda bottle and add 16.0 mL of
6-M HCl. Fill almost to the rim with water. Drop 1 tablet of AlkaSeltzer™ into this solution and stir until all bubbling has stopped.
The 6 M HCL is corrosive. Be careful not to spill any on your
clothes or skin. In the event of such a spill wash the area with
plenty of water and notify your instructor immediately.
2. Obtain a gas buret and fill it with the above acidified water. Invert into
the 2-L soda bottle and clamp as shown in the figure on the next page.
Be certain there are no air bubbles inside the gas buret before you
proceed.
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7
3. Clamp a 125 mL Erlenmeyer flask unto a ring stand. Obtain the glass
tubing in rubber stopper and a length of rubber tubing. Complete the
connections as shown in the diagram and image below. As you insert
the tubing into the buret, allow some liquid to flow into the tubing.
gas buret
clamp
rubber tubing
stopper with glass
tubing
2L
bottle
125 mL Erlenmeyer flask
ringstands
4. Pour 20 mL of 6-M HCl into the Erlenmeyer flask.
5. Weigh an Alka-Seltzer™ tablet. Crush it using a mortar and pestle.
Weigh three empty gelatin capsules, partially fill each capsule with
some crushed Alka-Seltzer,™ and re-weigh. The difference in weight
represents the amount of sample you will use in the trial. The amount
of Alka-Seltzer™ in each capsule should be between 0.25 and 0.30
grams (weighed to the nearest 0.001 g). Place each sample-containing
capsule into a separate, dry, labeled container so that you know which
sample is which weight and record the masses of your samples in your
lab notebook.
6. The next step is critical and must be performed quickly and with
precision. Remove the stopper from the reaction flask. Drop one of the
capsules containing the Alka-Seltzer™ into the flask and quickly
replace the stopper. Make sure the flask is tilted (as in diagram) so
that the capsule is immersed in the acid. The reaction will not begin
until the gelatin capsule has dissolved.
7. As the reaction proceeds, the level of water in the gas buret will drop.
When it appears that the reaction is complete, gently agitate the
reaction flask to complete the reaction and free any trapped gas.
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Alka-Seltzer Analysis
8. Adjust the gas buret by raising it up or down until the level of water
inside is exactly equal to the level of water in the soda bottle. Be
careful not to remove it from the water or the gas will escape! This step
is important, as there is no other easy way to determine the pressure of
the gas inside. When the water levels are equal, we know the pressure
inside the gas buret is the same as the pressure on the outside that
can be read from a barometer in the lab. Read the volume of the gas at
the adjusted position and record in your notebook. (If you cannot level
the water in the soda bottle set up, obtain a large graduated cylinder
for this purpose.)
9. Empty the contents of the buret into the sink and refill the buret with
acidified solution, as you did in step 2 above.
10. Repeat steps 6-9 for the other samples of Alka-Seltzer, but do not
change the acid in the Erlenmeyer flask (it is in great excess).
11. Finally, record the temperature of the acidified water and the
barometric pressure (assume that these are constant for all trials).
You will also need to look up the vapor pressure of water for the
recorded temperature.
12. Dispose of the solution in your soda bottle in the sink, clean all your
materials and return them to the appropriate locations.
Analysis:
Complete the calculations described below in order to determine the
sodium hydrogen carbonate content in the Alka-Seltzer™ tablet. You
may want to utilize the data template (available in the same facdata
folder as last week’s report sheet was) that accompanies this lab as you
perform the calculations.
1. The gas that you collected was a mixture of water vapor and carbon
dioxide. Determine the partial pressure of the carbon dioxide gas for
each trial by using Dalton’s Law.
Recall that the total pressure was
set to equal the barometric pressure of the room. You will need to use
the vapor pressure of water for the recorded temperature.
1.00 atm = 760 mmHg = 1013.25 mBar
2. Using the Ideal Gas Law, calculate the number of moles of carbon
dioxide collected in each trial and then the moles of sodium hydrogen
carbonate consumed in the reaction.
3. Compute the mass of sodium hydrogen carbonate ion present in each
sample.
4. Use the mass of sodium hydrogen carbonate present in each sample to
calculate the mass percent of sodium hydrogen carbonate in each
sample. Average your three results.
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9
5. Using the total mass of the tablet and the average mass percent of
NaHCO3, calculate the mass (in mg) of NaHCO3 per tablet.
6. Calculate the percent difference between your value and the value
reported on the package.
Lab Report: You will write a complete, formal lab report for this experiment. It
should include a creative title, a statement of purpose, a summary of
the methods used in the experiment, a table of the data collected,
examples of the calculations used to convert your raw data to results, a
table of results, and a discussion section that addresses the following:

As mentioned before, Alka-Seltzer™ also contains citric acid as an
active ingredient. Can you explain why this acid does not simply
react with the bicarbonate in the tablet while the tablet is still in
the package? As a hint – look at the neutralization reaction and
consider the phases that are indicated versus those of the reactants
while they are in the tablet inside the package.

Does carbon dioxide dissolve in water? If so, perhaps you can
explain why you had to drop 1 tablet of Alka-Seltzer™ into the 2L
soda bottle before starting the collection of carbon dioxide in the
gas buret.
If you worked with a partner, you must each hand in separate reports;
your partner’s name must be included in your report.
Prior to coming to the next lab, be sure to complete the pre-lab exercises for the
Enthalpy of Vaporization of Water experiment. As you hopefully did for this
experiment, you should write out your solutions to these questions in your lab
notebook, so that you have a carbon copy of your work in case you want to refer to it
as you do your calculations or if it should get lost after you hand it in.
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Alka-Seltzer Analysis