Bellevue College CHEM& 121
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Experiment: Stoichiometric Analysis of an Antacid1
Introduction
In this lab, you will use the concept of stoichiometry to solve two sequential problems. First, you will
try to determine the products of a certain reaction (below), choosing between three possibilities. Then,
you’ll use your results of this first part to determine the amount of sodium bicarbonate in a common
household substance.
CH3COOH (aq) + NaHCO3(s) → CO2(g) + ???
You’ve probably seen this reaction in elementary school -- add a few drops of red food coloring, and
you have the classic “volcano reaction.” Or, you can perform it easily in your kitchen by mixing
vinegar (dilute acetic acid) and baking soda (sodium bicarbonate). The most noticeable sign of the
reaction is vigorous bubbling, a result of very rapid carbon dioxide generation.
Gaseous carbon dioxide is one of the products, as you can see with your own eyes. (You can prove the
gas to be carbon dioxide by collecting it in a flask, and inserting a burning match into the flask. The
flame will be immediately extinguished.) Aside from carbon dioxide, what else is produced by the
reaction?
A chemist approaching this problem would most likely form some hypotheses about the other
products, and then design experiments to evaluate which hypothesis is best supported by experimental
evidence. For this experiment, we’ll supply three possible reactions, shown below. Notice that they are
all balanced equations.
A. CH3COOH (aq) + NaHCO3(s) → 2 CO2(g) + CH2O(aq) + Na+(aq) + 3H+(aq)
B. CH3COOH (aq) + NaHCO3(s) → CO2(g) + H2O(l) + CH3COO-Na+ (aq)
C. CH3COOH (aq) + 2 NaHCO3(s) → CO2(g) + Na2CO3(aq) + H2O(l) + 2 CH2O(aq)
Your job is to determine which of these three possibilities is correct, using some simple laboratory
measurements. Focus on the differences between the three proposals: Reactions A and C produce
formaldehyde (CH2O), but Reaction B doesn’t. The products of Reaction A are acidic (H+ is produced);
those of B and C are basic (CH3COO-Na+ and Na2CO3 are produced). These things could be tested, but
an even simpler method would be take advantage of the different amounts of carbon dioxide
produced, relative to sodium bicarbonate:
In Reaction A, 1 mole NaHCO3 produces 2 moles CO2
In Reaction B, 1 mole NaHCO3 produces 1 mole CO2
In Reaction C, 2 mole NaHCO3 produces 1 mole CO2
You will measure the ratio of moles NaHCO3 used to moles of CO2 produced, and if it is approximately
1:2, you may conclude that Reaction A is correct; if the ratio is around 1:1, you can bet that Reaction B is
correct, and if it’s about 2:1, you should choose Reaction C.
1 Used%with%permission%from%Seattle%Central%Community%College,%2011%
