The Determination of Bicarbonate
in an Antacid
I. Objective:
*To apply the ideal gas law to calculate the percent of
bicarbonate in an antacid tablet.
*To handle gases in a lab setting and study their properties.
II. Safety Issues:
Make sure that your instructor has checked your set-up
before beginning the reaction. Concentrated HCl is a
contact hazard; immediately flush skin or eyes if contact
occurs. Dispose of all solutions in the sink when you finish.
III. Introduction:
Calcium carbonate is found in limestone and many marine
shells. It is the main source of industrially important
carbonate salts like sodium bicarbonate (baking soda) and
sodium carbonate (soda ash). All carbonate salts react with
acid to form carbon dioxide. Antacids usually contain a
carbonate or bicarbonate salt; Alka Seltzer contains the
latter.
Sodium bicarbonate produces carbon dioxide when reacted
with HCl in the following reaction:
NaHCO3(s) + HCl(aq)  CO2(g) + H2O(l) + NaCl(aq)
You will be generating CO2 gas be the reaction of HCl with
an Alka-Seltzer tablet. Other antacids react similarly with
acid. The primary equation that is used to describe the
behavior of gases is the ideal gas equation: PV = nRT
IV. Laboratory Procedure
1. Assemble your gas-collecting apparatus as shown by your
instructor. Be very careful when inserting the glass tubing
into the hoses.
2. Fill the water trough about 2/3 full with water and make
sure the trough does not leak. Also completely fill a gas
collection bottle with water. There should be no air bubble
present in the jar. Cover the jar with a watch glass and
quickly invert the jar below the surface of the water in the
trough.
3. Connect a piece of rubber tubing to one end of the Jshaped piece of glass tubing. Then insert the other end into
the inverted gas collection bottle.
4. Connect the other end of the rubber tubing to the glass
tubing that is inserted into the rubber stopper that is on
top of the reaction bottle. Make sure the tubing is very
secure.
5. STOP. Have Mrs. Kuhn check your apparatus at this point
before beginning your reaction.
6. After Mrs. Kuhn has approved your set-up, measure out
10 mL of 6M HCl and pour this into the reaction jar.
7. Weigh out 1/2 of an Alka-Seltzer tablet using a forceps
to pick up and transport the tablet. Do not touch it! Record
this mass to 0.1 grams.
8. Using the forceps, drop the tablet into the reaction
bottle and as quickly as possible, place the rubber stopper
tightly onto the top of the jar. Observe the gases collected
in the gas collection bottle.
9. When the reaction has completely stopped (no more gas is
being produced), remove the J-shaped tube from the gas
collection jar and take the tubing out of the trough.
10. Balance the water levels inside and outside the gas
collection jar by raising or lowering the jar in trough. Once
the water levels are equal, place a watch glass over the
opening and quickly invert the bottle right side up and
remove from trough.
11. Using a graduated cylinder, measure the amount of
water required to completely refill the gas collection jar.
Record this amount in your data table.
12. Record the water temperature and the barometric
temperature.
13. Repeat the procedure and record the results in your
data table under trial 2.
14. Disassemble and clean your apparatus. Wash the
contents of the reaction flask down the sink.
2. Using Dalton's Law of Partial Pressures, find the
pressure of the dry CO2.
3. Using the mm Hg value for R, 62.4 L-mm Hg/mol-K, solve
for the moles of CO2 produced using PV = nRT.
4. Using the moles of CO2 from #3 and the molar mass of
NaHCO3, use stoichiometry to determine the number of
grams of NaHCO3 originally present in the tablet.
5. Calculate the mass percent of NaHCO3 in the tablet by
dividing the mass of NaHCO3 (#4) by the mass of antacid
tablet in your data table.
6. Obtain the theoretical mass percent from Mrs. Kuhn and
calculate your percent error.
V. Data
Trial 1
Trial 2
Mass of antacid tablet, g
Volume of tap water added, ml
Water temperature, ºC
Barometric pressure, mm Hg
Molar mass of NaHCO3
Vapor pressure of H2O, mm Hg
VI. Problem Solving: All work must be in lab book.
1. Using the values listed below, find the vapor pressure of
water for the temperature you measured and record it in
your data table.
Percent error = (theoretical yield - actual yield)/theoretical
yield x 100
Vapor Pressure of Water, mm
15ºC
12.79
23ºC
16ºC
13.63
24ºC
17ºC
14.53
25ºC
18ºC
15.48
26ºC
19ºC
16.48
27ºC
20ºC
17.54
28ºC
21ºC
18.65
29ºC
22ºC
19.83
30ºC
Hg
21.07
22.38
23.76
25.21
26.74
28.35
30.04
31.82