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Chemistry 121
Lab 2: Determination of the mass percent of sodium bicarbonate in Alka-Seltzer tablets
Objective:
To determine the amount of NaHCO3 in Alka-Seltzer tablets by observing the amount of CO2
produced from the acid-base reaction of HCO3- with acetic acid (in vinegar).
Background:
Alka-Seltzer is an effervescent tablet that contains aspirin (acetylsalicylic acid), citric
acid, and sodium bicarbonate (NaHCO3). As soon as the tablet dissolves in water, the NaHCO3
dissociates to form a bicarbonate ion (HCO3—) and a sodium ion (Na+)
NaHCO3 (s) → Na+ (aq) + HCO3— (aq)
Acetic acid, which will “donate” H+ to water to create H3O+(aq), is added to the mixture. The
following acid-base reaction then takes place:
HCO3— (aq) + H3O+ (aq) → 2H2O (l) + CO2 (g)
(The sodium ion does not take part in this reaction, and so is not included in the equation.)
According to the above equation, one mole of HCO3- reacts with one mole of H3O+. If
the number of moles of each reactant is not the same, than one reactant will be completely
used up during the reaction, while some of the other reactant will remain at the end of the
reaction. In this experiment you will dissolve Alka Seltzer in various concentrations of vinegar
and measure the amount of CO2 produced. From this data you will calculate the amount of
NaHCO3 in the Alka Seltzer by percent weight (mass).
Pre-lab Questions (please answer on your own paper):
1. What is the ratio of moles of CO2 produced to moles of NaHCO3 reacted? To phrase
it another way, for every mole of NaHCO3 reacted, how many moles of CO2 are
produced?
2. Mass can neither be created nor destroyed. If the mass of the solution (that is, the
combined mass of the beaker, water and tablet) decreases after the bubbles stop
forming, where did this “lost” mass go?
3. Why doesn’t it matter that the beaker is not completely dry after Run #1? Doesn’t this
throw off the mass that you are weighing?
4. How might this product (Alka-Seltzer) reduce indigestion or heartburn?
Explain your answer using the two chemical equations above.
Procedure:
1. Using a 50 mL graduated cylinder, measure about 35 mL of distilled water and record the
precise volume in the appropriate cell in column 1 (Run #1) in the data table (don’t just write
35 mL – you should read this number to the tenths of mL, just like in Lab 1).
2. Pour the water into a clean 250 mL beaker. Weigh and record the total mass of the beaker
plus water (still under Run #1 in the data table, and pay attention to significant figures).
3. Weigh and record the mass of an Alka-Seltzer tablet.
4. Drop the tablet into the beaker and swirl carefully. Record the length of time until the
bubbling ceases. After the bubbling ceases, weigh the beaker with the dissolved Alka-Seltzer
solution. Record this mass.
5. Rinse the beaker with distilled water. For Run #2, mix together about 5 mL of vinegar and
about 30 mL of distilled water (again, record each volume to its proper precision). Pour this
into the beaker, then weigh and record the mass of another Alka-Seltzer tablet, then drop the
tablet into the beaker, wait for the bubbling to cease, then re-weigh the beaker and its contents.
6. Repeat step 5 for subsequent runs, increasing the volume of vinegar by 5 mL each time
while keeping the total volume of the solution at 35 mL; Run #8 should be 35 mL of vinegar
with no water added. Record all your data in the table. Again, be sure to record the actual
measured volumes and record them with the correct number of significant figures for the
glassware that you use.
Chemistry 121
Name:
Lab 2 Data Sheet
Manufacturer % mass by weight of NaHCO3 in a tablet __________
Run #1
Volume of
vinegar (mL)
Volume of
water (mL)
Mass of beaker
with liquid (g)
Mass of Alka
Seltzer tablet (g)
Length of time
of the reaction (s)
Mass of beaker
with liquid after
bubbles cease (g)
Calculated mass
of CO2 lost (g)
Moles of CO2 lost
Moles of NaHCO3
reacted
Calculated mass
of NaHCO3
reacted (g)
Calculated % by
mass of reacted
NaHCO3 in tablet
Run #2
Run #3
Run #4
Run #5
Run #6
Run #7
Run #8
0.0
0.0
Post-lab analysis and questions:
1. What did you notice about the length of time it took for the bubbling to end, as the amount
of vinegar increased? What is the reason for this change?
2. How did the percent mass of reacted NaHCO3 change, as the amount of vinegar increased?
What is the reason for this change? Hint: you may want to use the phrase “limiting reactant”
in your answer.
3. In which runs do you think all of the NaHCO3 in the Alka-Seltzer tablet reacted? On what
do you base your conclusion?
4. Show, in detail, the calculations for run #4 in the data table, for all the bold-faced rows.
5. Based on your data and your answer to question 3, what is the average percent by mass of
NaHCO3 in an Alka Seltzer tablet? Show your calculation, and pay attention to significant
figures.
6. How much sodium bicarbonate is in Alka-Seltzer, according to the manufacturer? Pay
attention to units. Then show how you calculate the percent weight by mass of
NaHCO3, and add this value to the top of the data sheet.
7. Calculate the percent error between your average percent by mass of NaHCO3 and
the manufacturer’s value. Is this experiment an acceptable way of determining the
average percent by mass of NaHCO3 in an Alka-Seltzer tablet?