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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.
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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
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
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.
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.
waste in the labeled waste container.
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Report Name _____________________Section______ Stoichiometric Analysis of an Antacid Lab Partner ____________________________
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:
Prelab Name _____________________Section______ Stoichiometric Analysis of an Antacid