Determination of Percent By Mass of NaHCO3 in Alka Seltzer Tablets Class Notes

Alka Seltzer is an over the counter drug used to treat acid indigestion and pain. It contains aspirin (acetylsalicylic acid), sodium bicarbonate (NaHCO3) and citric acid. When you drop an alka seltzer tablet in water, an acid-base reaction occurs between the citric acid in the tablet and some of the sodium bicarbonate. This reaction produces effervescence with the produced gas CO2 causing the bubbling you see. What you have in the container of water after the fizzing stops are aspirin to treat the pain and some sodium bicarbonate to treat the acid symptoms. When the CO2 gas is released into the atmosphere a weight loss occurs. .

In this experiment we will still dissolve the alka seltzer in water, but because the tablet contains an excess of sodium bicarbonate we will use a vinegar-water solution to neutralize all the remaining sodium bicarbonate in the tablet. As we start this experiment the vinegar (acetic acid) is the limiting reagent and the sodium bicarbonate will be in excess. As we continue to add additional vinegar (acetic acid, HC2H3O2) more and more CO2 gas will be produced until we reach a point where no additional CO2 gas will be produced because the amount of sodium bicarbonate in the tablet is fixed and has been used up. At this point our vinegar (acetic acid) is now in excess and the sodium bicarbonate becomes our limiting reagent.

Limiting Reagent

The reactant that is completely consumed in a chemical reaction is called the limiting reactant (or limiting reagent) because it determines (or limits) the amount of product formed. The reaction will stop when all of the limiting reagent is consumed.


NaHCO3 + HC2H3O2 ---> H2O (l) + CO2 + NaC2H3O2

In the above reaction one mole of sodium bicarbonate reacts with one mole of acetic acid (vinegar) producing one mole of water, one mole of CO2 and one mole of NaC2H3O2.

STEP 1: Calculate the mass of CO2 generated when vinegar and alka seltzer are mixed.

STEP 2: Calculate the mass of NaHCO3 (sodium bicarbonate) reacted when CO2 is produced.


Using the above reaction, for every one mole of CO2 gas produced one mole of NaHCO3 (sodium bicarbonate) is reacted.

Because NaHCO3 is fixed (our limiting reagent when we used excess vinegar) the mass of NaHCO3 reacted is the mass of NaHCO3 in the tablet.

STEP3: Calculate the percent by mass of NaHCO3 (sodium bicarbonate) in the alka seltzer tablet.

Sample Problem

A tablet of Alka Seltzer weighs 4.05g. It is placed in a cup of liquid weighing 48.72g. After the tablet dissolves & effervescence occurs the cup with liquid and tablet weighs 51.26g. How much CO2 was generated?

Graphing a Curve of Best Fit

You will take your calculated data and plot the percent by mass of reacted sodium bicarbonate in a tablet versus the volume of vinegar used. You do not connect the points plotted. Rather you draw a best fit curve with the curve leveling off as the volume of vinegar increases. As the curve rises the percent by mass of sodium bicarbonate is in excess and the vinegar is the limiting reactant because all the vinegar is being used up during the reaction. When the curve levels off the percent by mass of sodium bicarbonate becomes the limiting reactant because it is a fixed amount in the tablet and the vinegar becomes in excess. Using the example graph below , look at where the curve levels off. You can then determine the average percent by mass of sodium bicarbonate in the tablet is 60%.

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last updated: March 3, 2015