Experiment 2-102a: Determination of Percent Mass of NHCO3 in Alka Seltzer Tablets.
Report Sheet for experiment 2-011c: Extra report sheet (pdf format)
The Concept:
This week you will experimentally determine the percent by mass of sodium bicarbonate in an Alka Seltzer tablet. Alka Seltzer is an over-the-counter antacid that contains aspirin, citric acid, and sodium bicarbonate. For maximum efficacy, the medication should be dissolved in water before it is ingested. As the tablet dissolves in water, sodium bicarbonate undergoes the acid-base reaction with the acids (citric and acetylsalicylic) contained in the tablet:
HCO3-+(aq) + H+(aq) ------> H2O (l) + CO2(g) [1]
The gas product, CO2, causes the bubbling. The release of carbon dioxide gas results in a weight loss after the reaction. According to the mole-mass relationship, the quantity of sodium bicarbonate that reacted can be calculated using weight loss .
The Experiment:
For this experiment, the mass percentage of NaHCO3 in an Alka Seltzer tablet will be analyzed by measuring the weight loss of the Alka Seltzer tablet when reacted with vinegar (ca. 5% acetic acid). By incrementally increasing the amount of acid (vinegar) used in each run of the reaction, one can observe the effect of the limiting reagent.
You will work in pairs for this experiment. You are going one start by dissolving a tablet (of known mass) in a beaker of pure water (which has also been massed). The sodium bicarbonate is in excess, and the acid (H+) is the limiting reactant so only a limited amount of NaHCO3 will be reacted. After the tablet has dissolved and all the gas has evolved you will weight the beaker and the Alka Seltzer solution.
You will then dissolve another tablet (of known mass) in a vinegar-water solution. The amount of acid (H+) has increased so more NaHCO3 will react. Once the tablet has fully dissolved and all the gas has been evolved, you will mass the beaker with the Alka Seltzer solution. With increasing amounts of vinegar in the vinegar-water solution, more NaHCO3 reacts and more CO2(g) is evolved. Eventually, the acid (H+) will be in excess, and since the NaHCO3 contained in the Alka Seltzer tablet is fixed, no additional CO2 will be generated with increasing amounts of vinegar. By calculating the loss of mass after each reaction, one should be able to calculate the amount of CO2 and therefore, the amount of sodium bicarbonate reacted. You can then determine the percent by mass of NaHCO3 contained in Alka Seltzer tablets.