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Stoichiometry Analysis: Measuring CO2 Production in Antacid Reaction, Study notes of Stoichiometry

In this document, students are provided with a lab experiment to determine the stoichiometry of a reaction between acetic acid and sodium bicarbonate using the production of carbon dioxide as an indicator. The experiment involves measuring the mass of the reaction mixture before and after the reaction, calculating the mass of carbon dioxide produced, and comparing the results to the given chemical equations to identify the correct reaction. Students will also apply their knowledge to measure the amount of sodium bicarbonate in Alka-Seltzer tablets.

What you will learn

  • What is the mole-to-mole ratio of sodium bicarbonate to carbon dioxide in the correct reaction?
  • Which reaction (A, B, or C) produces the most carbon dioxide when 1 mole of sodium bicarbonate is used?

Typology: Study notes

2021/2022

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Bellevue College CHEM& 121
Page 1 of 8
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%
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Experiment: Stoichiometric Analysis of an Antacid

1

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. CH 3 COOH (aq) + NaHCO 3 (s)CO 2 (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. CH 3 COOH (aq) + NaHCO 3 (s) → 2 CO 2 (g) + CH 2 O(aq) + Na+(aq) + 3H+(aq) B. CH 3 COOH (aq) + NaHCO 3 (s) → CO 2 (g) + H 2 O(l) + CH 3 COO

  • Na + (aq) C. CH 3 COOH (aq) + 2 NaHCO 3 (s) → CO 2 (g) + Na 2 CO 3 (aq) + H 2 O(l) + 2 CH 2 O(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 (CH 2 O), but Reaction B doesn’t. The products of Reaction A are acidic (H+^ is produced); those of B and C are basic (CH 3 COO-Na+^ and Na 2 CO 3 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 NaHCO 3 produces 2 moles CO 2 In Reaction B, 1 mole NaHCO 3 produces 1 mole CO 2 In Reaction C, 2 mole NaHCO 3 produces 1 mole CO 2 You will measure the ratio of moles NaHCO 3 used to moles of CO 2 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

Keep in mind that your results may not give you exact whole number mole-to-mole ratios because of basic experimental errors. Your results may be off by as much as 20% for this experiment, but you will still be able to choose between the three reactions (A, B, or C) with a fair amount of confidence if you work carefully and collect good data. Determining the moles of NaHCO 3 is easy: Use the measured mass you scoop out of the container to use. (The other reactant, acetic acid will be used in excess, so its exact amount will have no relationship to the amount of carbon dioxide generated.) Determining the moles of CO 2 is less straightforward; it’s not so simple to collect and measure the mass of a gaseous substance, as you can imagine. In each of the three reactions above, carbon dioxide is the only gas, and all other reactants and product are liquids, solids, or aqueous. As the reaction occurs, carbon dioxide will bubble out of the reaction solution and escape into the laboratory. Therefore, the mass of your reaction mixture after the reaction will be lighter due to the loss of carbon dioxide, and a simple subtraction tells you how much carbon dioxide was produced: ⎟ ⎠

afterreaction massofreaction mixture before reaction massofreaction mixture

Mass of CO 2 -

One small complication is that some of the CO 2 produced will remain dissolved in the reaction mixture because carbon dioxide is somewhat soluble in water. This means that the mass you calculate by subtraction in the above equation is somewhat too low—i.e., you have not accounted for the carbon dioxide that goes into the water. You will account for this with a correction factor in your calculations. Once you have chosen the correct reaction between acetic acid and sodium bicarbonate, you can use it to measure the amount of sodium bicarbonate in Alka-Seltzer tablets using a similar methodology. In this case, the mass of sodium bicarbonate will be an unknown. You can measure the amount of CO 2 produced as you did before, and use the mole-to-mole ratio of the chosen reaction to calculate the number of moles and the mass of sodium bicarbonate which reacted. Finally, you will determine your experimental error by comparing your experimentally determined mass of sodium bicarbonate present with what the manufacturer reported on the package of Alka-Seltzer.

Objectives

In this experiment, you will ü Determine the stoichiometry of a reaction experimentally. ü Weigh by difference a reaction mixture before and after the reaction in order to find the mass of a gas produced. ü Practice molar mass and mole ratio calculations. ü Calculate a percent error and determine how an inaccuracy in a specific measurement affects the outcome.

Hazards

M Even though the reagents in this lab are fairly safe, please wear safety goggles and dispose of

waste in the labeled waste container.

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Report Name _____________________Section______ Stoichiometric Analysis of an Antacid Lab Partner ____________________________

Data, Calculations, and Results

Part A: Reaction Stoichiometry Trial 1 Trial 2 Average Mass of NaHCO 3 Mass of beaker + acetic acid + NaHCO 3 + watch glass before reaction Mass of beaker + reaction mixture + watch glass after reaction Mass of CO 2 gas released Moles of CO 2 gas released Moles of CO 2 , corrected for amount dissolved* (see below) Moles of NaHCO 3 used NOTE: *Calculate by adding 0.0040 moles to the “Moles of CO 2 gas released”. This correction accounts for the amount of CO 2 that dissolves in 50 mL of aqueous solution. Calculate (and show your work for at least trial #1) for the following:

  1. The number of moles of CO 2 for Trial #1, 2, and 3.
  2. The number of moles of CO 2 , corrected, for Trial #1, 2, and 3.
  3. The number of moles of NaHCO 3 for Trial #1, 2, and 3.
  1. The corrected moles of CO 2 released by the tablet.
  2. Show the calculation for the moles of NaHCO 3 , using the mole ratio in the balanced chemical equation determined from Part A.
  3. Show the calculation for the mass of NaHCO 3 for the tablet.
  4. Show the calculation for the average mass of NaHCO 3 for the two tablets.
  5. The mass of sodium bicarbonate in each tablet of Alka-Seltzer is reported as 1916 mg on the package (which we will call the “actual value”). Using your experimental value for the average mass of NaHCO 3 , calculate the percent error. Show your work. 100% actual value |actualvalue-experimentalvalue | % error= ×
  6. In Part B, suppose the tablet was mostly dissolved when some of your solution splashed out of the beaker as CO 2 continued to evolve. How would this affect the perceived mass of CO 2? Would your final calculated mass of sodium bicarbonate in the tablet be artificially high or artificially low as a result of this splashing? Choose one, and explain why.

Prelab Name _____________________Section______ Stoichiometric Analysis of an Antacid

  1. When acetic acid and sodium bicarbonate react, why does total mass decrease?
  2. Suppose you are running a trial of the reaction in Part A. You use 2.80 g of NaHCO 3 , and determine that 2.83 g of carbon dioxide are produced. Using these data and a periodic table, show calculations to: a. Convert the mass of NaHCO 3 to moles using the molar mass of NaHCO 3. b. Convert the mass of CO 2 to moles using the molar mass of CO 2. c. Find the simplest ratio between the number of moles of NaHCO 3 and CO 2. ( Hint: Divide both numbers by the smaller of the two to get a ratio .) The mole ratio of NaHCO 3 : CO 2 is ____:____. d. Compare this ratio to the ratio of the coefficients for NaHCO 3 and CO 2 in the three balanced chemical equations (given on page 1). Which of the three possible reactions discussed is consistent with these results? WRITE THE EQUATION HERE. NOTE: The result in part C is not necessarily the correct answer. It is a hypothetical situation for practicing the calculations required in this lab. Do not use this actual result in the lab!