This is an example of a good lab explanation
Conclusion:
Based on experimental results of CO2 production, the amount of H+ present increases the production of CO2
by an overall 106% due to the increase of acidity from the vinegar solutions. The percent by mass of the NaHCO3 in
each run increases by 102% as the amount of vinegar used in solution increases.
Analysis:
Alka Seltzer, containing acetylsalicylic acid (HC9H7O4), citric acid (HC6H7O7), and sodium bicarbonate
(NaHCO3), is an antacid and analgesic medication that dissolves when in contact with water, emitting “bubbles”
identical to carbonation. The tablet when dissolved in water undergoes an acid-base reaction where the sodium
bicarbonate reacts with the acetylsalicylic and citric acids, resulting in the reaction of HCO3- (aq) + H+ (aq) H2O (l) +
CO2 (g). The carbon dioxide gas produced causes the “bubbling” of the tablet and the loss of the CO2 gas results in a
difference of mass at the end of the reaction. By using the mass difference the percent by mass of sodium
bicarbonate that had reacted in the Alka Seltzer tablet can be calculated.
When an Alka Seltzer tablet is dissolved in a water solution the sodium bicarbonate is in excess and the H+
from the acid is the limiting reagent (reference page 2). Only a portion of sodium bicarbonate can react due to the
limit of H+ in the reaction. When using a vinegar-water solution instead, more acid is available to the reaction and
more sodium bicarbonate is able to react, thus producing more CO2. With increasing volumes of vinegar more CO2
is produced and eventually the H+ will become the excess reagent and sodium bicarbonate the limiting reagent. The
amount of sodium bicarbonate in each tablet is the constant 1916 mg so additional CO2 will not be produced as
increasing volumes of vinegar are added to the solution.
Eight runs were performed during this experiment with a consistent volume of 35 mL solution. The first
run, acting as a control, consisted of 35.0 mL of water and 0 mL of vinegar had an initial mass of 142.47 g and a
final mass of 145.05 g. By using the difference of mass the grams of CO2 lost can be calculated, which was about
0.67 g of CO2 lost in the reaction (reference calculations on page 5). The second run consisted of 30.0 mL of water
and 5.00 mL of vinegar which massed to 143.03 g and a final mass of 145.01 g. The CO2 lost was approximately
0.85 g. The amount of vinegar used in each run increases by regular increments of 5.0 mL. As more vinegar is used
there is a larger amount of CO2 being released as the increase of H+ from the vinegar is available to drive the
reaction to completion, being able to produce more products (CO2). Observation of the trend on the graph shows
the amount of CO2 released increases as the amount of vinegar used increases as well. There is a 26.9% increase of
CO2 between Runs 1 and 2 and a 25.4% increase between Runs 1 and 3. There is a 56.7% increase of CO2 between
Runs 1 and 4 and a 47.8% increase between Runs 1 and 5. Between Runs 1 and 6 there is a 53.7% increase, a
55.2% increase between Runs 1 and 7 and a 106% increase of CO2 between Runs 1 and 8.
The percent by mass of NaHCO3 in each tablet in each run conducted in this experiment increased as the
amount of vinegar used in solution increased as determined through stoichiometry relationships. Observation of
the trend shows a similar trend as the graph of CO2 released compared to the amount of vinegar used. In Run 1,
1.28 g of NaHCO3 was used and the percent by mass was 39.4%. In Run 2, the NaHCO3 used was 1.62 g and the
percent by mass was 50.2%, in Run 3 1.60 g and 49.2%, Run 4 2.00 g and 60.6%, Run 5 1.89 g and 57.6%, Run 6
1.97 g and 60.4%, Run 7 1.99 g and 60.7%, and Run 8 with 2.63 g used and a percent by mass of 79.7%.
A source of error in this lab would be if the Alka Seltzer tablet was not fully dissolved before final massing.
If the tablet was massed without fully dissolving then the amount of CO2 released is lower than the amount if the
tablet was fully dissolved, which explains the inconsistency of the trend of the graph where the increasing volume
of vinegar had a lower amount of CO2 lost, for example between Runs 2 and 3 and between Runs 4 and 5 (reference
page 3). Another source of error in this lab would be the difference in mass of the beakers used. Two different
beakers were accidently used during this experiment and the difference of mass of the beakers would have affected
the results of the lab when massing the initial and final masses of the different runs. Also, there is a possibility of an
inconsistency with 35.0 mL of solution as more or less could have been used with each run.