ELAC Chemistry Evaluating the Cost Effectiveness of Antacids Lab Report

Santa Monica CollegeChemistry 11
Evaluating the Cost-Effectiveness of Antacids
Objectives
In this experiment, two commercially available antacids are evaluated and compared by:
• determining the number of moles of H+ neutralized per gram of each antacid.
• calculating the cost effectiveness of each antacid.
Background
Acid indigestion is a common ailment caused by the overproduction of stomach acid, HCl.
Over-the-counter antacids provide some relief from the symptoms of acid indigestion. They are
generally made up of some mixture of weak bases such as Mg(OH)2, Al(OH)3, and CaCO3 that
can react with HCl as shown in these net ionic equations:
H+ (aq) + OH– (aq) → H2O (l)
2 H+ (aq) + CO32– (aq) → H2O (l) + CO2 (g)
In this experiment, a titration will be used to determine the number of moles of H+ neutralized
per gram of antacid. First, a pre-weighed sample of antacid will be mixed with an excess of
HCl. Then the H+ that has not reacted with the antacid is titrated with standardized NaOH in the
presence of the indicator bromophenol blue to a blue-gray end point.
The end point, is defined as the volume of OH– needed to see a color change. Because only
the tiniest excess of OH– over H+ can cause the color change of an indicator, the end point is a
close approximation of the equivalence point. At the equivalence point, the number of moles
of OH– added is equal to the number of moles of excess H+ that had not been neutralized by the
antacid. By knowing the total moles of HCl added, the number of moles of H+ neutralized by the
antacid can thus be calculated:
Total moles of H+ used = moles of H+ neutralized by antacid + moles of H+ neutralized by NaOH
Because the antacid includes both OH– and CO32–, it is not possible to calculate the number of
moles of each of these ion species independently. Instead, the number of moles of H+ neutralized
by the antacid is determined. The amount of antacid required to neutralize one mole of H+
neutralized is said to be one “equivalent.”
total equivalents of antacid = total moles of H+ neutralized by antacid
The more cost-effective antacid is the one that costs fewer dollars per equivalent.
Evaluating the Cost-Effectiveness of Antacids
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Santa Monica College
Chemistry 11
Procedure
Safety
 Take care in handling both HCl and NaOH. Should either of these solutions come in contact with
skin or eyes, rinse thoroughly with water.
 Allow enough time for the boiled antacid solution to come to room temperature before touching it.
Materials and Equipment
Chemicals: Antacid tablets (Rolaids ®, Tums ® etc. – it is preferable to use tablets that are white
and have no added flavor), 0.1 M standardized HCl, 0.1 M standardized NaOH, Bromophenol blue
indicator solution.
Equipment: Two burets*, mortar and pestle, two 250 mL Erlenmeyer flasks, two (or more) 100 mL
beakers, stand and ring clamp, wire gauze, Bunsen burner.
*Items with an asterisk must be checked out from the stockroom.
Instructions
Preparation of Antacid Sample
1. Choose an antacid and record its name on the data sheet. Using a mortar and pestle, crush
one tablet of antacid to as fine a powder as possible.
2. Weigh out between 0.3 – 0.4 g of the powdered antacid into a pre-weighed 250-mL Erlenmeyer
flask. Record the mass of the antacid sample to 0.001g.
Preparation of Burets
3. Obtain two burets from the stockroom and, with soap and water, wash them with a brush. Rinse
them well with deionized water (Note: when the buret is cleaned properly, there should be no
water droplets clinging to the inside of the buret). One buret will be filled with the standardized
HCl and the other with standardized NaOH. Label them accordingly and record their
concentrations as indicated on the stock solution labels.
4. It is important that the concentration of the acid and base are not diluted with any of the water
left over from cleaning. So, before filling the burets, rinse them well with at least two 4-mL
portions of either HCl or NaOH as appropriate.
5. Using a beaker, fill the burets with HCl and NaOH. Note that bubbles take up space, so make
sure to dislodge any bubbles that might be stuck in the tip of the buret by lightly tapping the tip
or by letting a portion of the solution run rapidly through the tip. Record the initial buret readings
for both HCl and NaOH.
Addition of Excess HCl to the Antacid
6. Record the initial volume of HCl in the buret and add approximately 40 mL of standardized HCl
to the prepared antacid sample. Record the final volume of HCl in the buret to 0.01 mL.
Evaluating the Cost-Effectiveness of Antacids
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Santa Monica College
Chemistry 11
7. In order to dissolve as much as of the antacid as possible and to drive off as much dissolved
CO2 as possible, gently boil the mixture of antacid + HCl for about two minutes. Cool the
mixture to room temperature. There may be a significant amount of undissolved substances
present in your mixture. Because the active ingredients of an antacid are quite water soluble,
these solids will not affect the results. The solids are a mixture of inactive ingredients such as
the coating and some binding compounds.
8. Add 6 – 8 drops of bromophenol blue indicator to the boiled solution. If there is excess H+
present then the solution will be yellow, and no more HCl is necessary. But if the solution is
green, blue, or a mixture of both green and blue, there is excess OH– in solution and more HCl
must be added. In this case add about 20 mL of HCl from the buret, again recording to 0.01
mL. Make sure there is enough HCl in the buret before doing this. Remember, it is not possible
to measure the volume of HCl if the level drops below the 50.00 mL mark! Then assess the
color of the solution. If it is not yellow, repeat the addition of HCl until it is.
Titration of excess HCl with NaOH
9. Begin the neutralization of the excess HCl with the addition of standardized NaOH. Record the
initial volume on the NaOH buret to 0.01 mL. Begin the titration by slowly adding the NaOH in 1
mL increments. Mix well between additions by swirling the flask.
10. As more and more NaOH is added, the solution will turn from yellow to green and then finally
blue-gray. The titration reaches the end point when the addition of one to three drops of NaOH
turn the solution blue-gray and that color is stable with mixing for at least 20 seconds. Record
the final buret reading and calculate the number of OH– moles added.
Back–Titration
11. This step is only necessary if too much NaOH has been added and the end point has
been exceeded. If your solution is a very bright blue you have added too much NaOH! Don’t
panic, this error can be corrected. Add enough of the standardized HCl to make the solution
yellow (about 5 – 8 mL). Record the new total volume of HCl. Again from the buret, add NaOH
slowly until the solution turns blue.
Repeat
12. Repeat this procedure with another brand of antacid. Make every effort to reach an endpoint
that has equivalent in its “blueness”.
Clean–Up
13. Before returning burets to the stockroom, rinse them thoroughly with deionized water. Also,
wash all other glassware with soap and water and rinse thoroughly with deionized water.
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