Alka-Seltzer® tablets contain the primary active ingredients of citric acid and sodium bicarbonate, — Laboratory / Stoichiometry Chemistry Question

• Mass of all of the items before mixing – making sure to include the beaker, watch glass, tablet, solution
• Mass of the items after mixing
• Allow time for the reaction to occur – stirring and waiting until no visible gas remains in the solution
• Indication of conditions under which the tablet will be reacted (water alone, with vinegar)

• Clear data table including starting and ending masses and the volumes of water/vinegar used with at least two separate experiments (one with water and one with vinegar) – duplicate trials
• Must react the tablet with water alone and then with at least enough vinegar that it is in the state where the sodium bicarbonate is the limiting reagent
• Observations – could include that the solution bubbled, the final masses were recorded after all visible gas was removed, a small amount of solid in the beaker (possibly the aspirin)

Experimental Calculations
The student should determine which active ingredient in the tablet is the limiting reagent, which in the tablet is the citric acid. Once the limiting reagent is determined, the most direct way to determine the amount of sodium bicarbonate is to react the tablet with excess vinegar and determine the mass of carbon dioxide lost, which is equivalent to the amount of bicarbonate present in the tablet. Dissolving the tablet in only water, allows the reaction to occur between the citric acid and the sodium bicarbonate in the tablet. As the citric acid is limiting, the mass of carbon dioxide lost is equivalent, as long as you consider the molar stoichiometry, to the mass of citric acid in the tablet.

H₃A + 3 NaHCO₃ → Na₃A + 3 CO₂ + 3 H₂O

Sample Student Data
Exp water (mL) acid (mL) mass tablet (g) mass CO₂ (g) mass NaHCO₃ reacted (g) mass %
1 35 0 3.227 0.603 1.151 35.67
2 30 5 3.229 0.818 1.562 48.35
3 25 10 3.244 0.912 1.741 53.67
4 20 15 3.234 0.972 1.856 57.41
5 15 20 3.231 0.980 1.870 57.88
6 10 25 3.254 0.999 1.909 58.57
7 5 30 3.266 1.018 1.943 59.50
8 0 35 3.263 0.955 1.822 55.73

For determining citric acid:
Exp water (mL) acid (mL) mass tablet (g) mass CO₂ (g) Mass of citric acid reacted (g) mass %
1 35 0 3.227 0.603 0.877 27.19

• Mass of carbon dioxide lost in each experiment
• Mass of sodium bicarbonate reacted
• Using the trial with the water, determine the mass of citric acid consumed
• Calculating the mass percentage of the sodium bicarbonate and the citric acid
• Accuracy of data

59.1% by mass (Actual composition of Alka Seltzer Tablets: 1916 mg sodium bicarbonate)

30.9% by mass (Actual composition of Alka Seltzer Tablets: 1000 mg citric acid)