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1 Stoichiometry The term "stoichiometry" refers to the quantitative relationships between reactants and products in chemical reactions. In a balanced chemical equation, the coefficients (or mole ratios) represent the stoichiometry of the reaction. Often reactions occur with one reactant in excess of the stoichiometric amount that is needed. The reactant not in excess is called the "limiting reagent". The limiting reagent is completely consumed during the reaction and limits the amount of product that is produced. Determining which reactant is the limiting reagent will allow the calculation of the theoretical yield. The theoretical yield is the maximum amount of a product that can be produced from the starting amounts of the reactants. The mass of each reactant will be measured exactly before the reaction, and the mass of one of the products will be determined at the end of the reaction. The mass of the product collected is called the "actual yield". Due to spillage, glassware, and side reactions, the actual (obtained) yield should be less than the theoretical yield. However if the product is not completely dry, the actual yield may be greater than the theoretical yield, resulting in a percent yield greater than 100 %. Today you are working at a pharmaceutical company that manufactures Over the Counter (OTC) medications that are used as antacids. Your boss has asked you to run two different experiments and to determine the percent yield for each reaction, so that the company can decide whether these procedures are feasible in the manufacturing of these OTC medications. In Part A of this experiment you will be reacting sodium carbonate with calcium chloride to produce a common substance that is marketed as an antacid. Can you predict the products of this reaction? (Try writing a balanced reaction on your data sheet.) This common substance has several names and uses. It is also found in Over the Counter calcium supplements to strengthen bones. What is the name of some OTC products that contain this substance? What do geologists call this substance when it is found in nature? In Part B of this experiment you will be reacting magnesium sulfate heptahydrate with sodium hydroxide to produce another OTC medicine that is also an antacid. Can you predict the products of this reaction? (Try writing a balanced reaction on your data sheet.) Do you know what this OTC medication is called? What is the name of some OTC products that contain this substance? Objectives: 1. Use the analytical balance and the tare function to determine an exact weight of a reactant. 2. Write a balanced molecular equation. 3. Use a Buchner funnel to filter a solution and isolate the solid. 4. Determine the molar mass of a compound.

2 5. Determine the limiting reagent and calculate the theoretical yield of a reaction. 6. Calculate percent yield. Prior to lab read the sections of our textbook that address the topics: stoichiometry, molar mass, limiting reagents, theoretical yield, percent yield. Chemicals: Solid Na 2 CO 3, CaCl 2, NaOH, and MgSO 4. 7H 2 O Acetone, R.O. Water Equipment: Oven Beakers Buchner funnel side-arm vacuum flask glass stirring rods petri dishes vacuum tubing Filter papers: size # 1 for calcium carbonate size # 4 for magnesium hydroxide) Sample Calculations: A CHM 120 student weighs out.600 grams lead (II) nitrate Pb(NO 3 ) 2 and.500 grams sodium iodide, NaI. Each reactant is dissolved in 50.0 ml of water, and then mixed together. Lead (II) iodide is produced as a solid and is filtered out of the mixture. The student collected grams PbI 2. The balanced reaction is written below. Pb(NO 3 ) 2 (aq) + 2 NaI (aq) PbI 2 (s) + 2 NaNO 3 (aq) In the balanced equation, lead nitrate and sodium iodide react in a 1:2 mole ratio to produce 1 mole of lead iodide. The mole ratios or coefficients of the balanced equation represent the stoichiometry of the reaction. To determine the theoretical yield of lead iodide PbI 2, the limiting reagent must be determined. First the number of moles of lead nitrate and the number of moles of sodium iodide must be determined, and then the number of moles of lead iodide produced from each reagent must be calculated.

3 Calculate moles of lead nitrate: Molar mass of Pb(NO 3 ) 2 = g / mole # moles Pb(NO 3 ) 2 =.600 grams x 1 mole / grams = moles Pb(NO 3 ) 2 Calculate moles of sodium iodide. Molar Mass of NaI = g / mole # moles NaI =.500 grams x 1 mole/ grams = moles NaI Determination of the limiting reagent and the theoretical yield. Pb(NO 3 ) NaI PbI NaNO 3 The mole ratio between Pb(NO 3 ) 2 and PbI 2 is 1:1. If moles of Pb(NO 3 ) 2 reacts moles PbI 2 forms The mole ratio between NaI and PbI 2 is 2:1. If moles NaI reacts moles PbI 2 forms. The number of moles of sodium iodide limits the amount of PbI 2 that is produced. Only moles PbI 2 forms. The limiting reagent is the sodium iodide. NaI determines the amount of lead iodide that can be produced. The lead nitrate is in excess, not all of it reacts. Some lead nitrate remains unreacted. Calculate theoretical yield of lead iodide. Molar Mass of PbI 2 = g / mole Theoretical Yield = moles PbI 2 x grams / mole PbI 2 Theoretical yield = grams PbI 2 Determination of % yield, based on actual yield. In this experiment, the actual yield of PbI 2 collected was grams.

4 Percent yield = % Yield = (Actual yield / Theoretical yield) x 100 = (.651 g /.770 g) x 100 = 84.5 % Download data spreadsheets Part A: Reaction of sodium carbonate and calcium chloride: Using chemical formulas, write a balanced equation for this reaction at the top of your data page. Include phases. sodium carbonate + calcium chloride --> calcium carbonate + sodium chloride 1. Weigh out approximately grams each of sodium carbonate and calcium chloride. a. Use the analytical balance, the tare function and a plastic weigh tray. b. Record the EXACT masses ( g) in the data table. 2. Add 25 ml of R.O. water to two clean 50 ml beakers. a. Add the sodium carbonate to one beaker and dissolve the solid by stirring with a stirring rod. b. Add the calcium chloride to the other beaker and dissolve the solid by stirring with a stirring rod. 3. Mix the solutions containing the dissolved sodium carbonate and calcium chloride. A cloudy white suspension will form. Filtering and collecting the product: 1. Obtain a piece of # 1 filter paper. RECORD the mass of the filter paper and a petri dish. (# 1 filter paper is used for filtering crystalline precipitates such as calcium carbonate.) 2. Filter the solid from the suspension. a. Attach a Buchner funnel to a vacuum flask. Use a ring clamp to secure it so that it does not tip over. b. Add the weighed piece of filter paper to the septum of the funnel. c. "Wet" the filter paper with a small amount of water. The paper should be secure in the filter, with no holes showing. d. Attach a vacuum hose and turn on the vacuum slowly. e. Pour the suspension slowly in the center of the filter paper. Increase the vacuum. f. Rinse any remaining solid from the beaker with some water. 3. Rinsing and collecting the calcium carbonate. a. Rinse the crystals with 10 ml of acetone to remove any water. b. Repeat the acetone rinse with another 10 ml. c. Allow the crystals to dry with the vacuum on for 15 minutes. d. Turn off the vacuum and disconnect the vacuum hose from the filter.

5 e. Using a flat spatula, carefully lift the filter paper and crystals. Place the filter paper and crystals in the previously weighed petri dish. f. Dry the crystals in the oven on the filter paper in the petri dish for 30 minutes, or longer until the paper and the crystals are completely dry. g. While the product is filtering and drying, you can calculate the limiting reagent and the theoretical yield of the product on your data sheet. Show your calculations. h. Allow the dish and crystals to cool for 5 minutes and then weigh it. 4. Dispose of the solid calcium carbonate in the waste basket. Calculate the % yield of the product using the actual yield and the theoretical yield. Record the % yield on your data sheet. Part B: Reaction of NaOH and magnesium sulfate heptahydrate: Safety Precautions: NaOH can be very corrosive and can cause severe burns. Do not touch solid NaOH! Keep safety glasses on at all times to prevent eye injuries. 1. Using chemical formulas, write a balanced reaction for this experiment at the top of your data page. Include phases. sodium hydroxide + magnesium sulfate heptahydrate --> magnesium hydroxide + sodium sulfate + water 2. Weigh out approx grams sodium hydroxide and grams magnesium sulfate heptahydrate. a. Use the analytical balance, the tare function and a plastic weigh tray. b. Record the EXACT weights ( g) in the data table. 3. Add 25 ml of R. O. water to two clean 50 ml or 100 ml beakers.. a. Add the sodium hydroxide to one beaker and dissolve the solid by stirring with a stirring rod. b. Add the magnesium sulfate heptahydrate to the other beaker and dissolve the solid by stirring with a stirring rod. 4. Mix the solutions containing the sodium hydroxide and magnesium sulfate heptahydrate. A cloudy white suspension will form. Filtering and collecting the product: 1. Obtain a piece of # 4 filter paper. RECORD the mass of the filter paper and petri dish to be used. (# 4 filter paper is used for filtering gel and coarse precipitates: magnesium hydroxide.) This suspension will filter slowly. 2. Filter the solid from the suspension. a. Attach a Buchner funnel to a vacuum flask. Secure it with a ring clamp. b. Add the weighed filter paper to the septum of the funnel.

6 c. "Wet" the filter paper with a small amount of water. The paper should be secure to the septum, with no holes showing. d. Attach a vacuum hose and turn on the vacuum slowly. e. Pour the suspension in the center of the filter paper. Increase the vacuum. f. Rinse any solid from the beaker with some water. 3. Rinsing and collecting magnesium hydroxide. a. Rinse the crystals with 10 ml of acetone to remove any water. b. Repeat the acetone rinse with another 10 ml. c. Allow the crystals to dry with the vacuum on for 15 minutes. d. Using a flat spatula, carefully lift the filter paper and crystals. Place the filter paper and crystals in a previously weighed petri dish. e. Dry the crystals in the oven on the filter paper in a petri dish for 30 minutes, or longer until the paper and the crystals are completely dry. f. While the product is filtering and drying, you can calculate the limiting reagent and the theoretical yield of the product on your data sheet. Show your calculations. g. Allow the dish and crystals to cool for 5 minutes and then weigh it. 4. Dispose of the solid magnesium hydroxide in the waste basket. 5. Clean all glassware and return all equipment to its correct location. Calculate the % yield of the product using the actual yield and theoretical yield values. Record the % yield on your data sheet. FOLLOW-UP QUESTIONS: 1. Pharmaceutical companies and chemical manufacturing plants try to obtain high percent yields of pure products. List and explain several sources of error you observed that may have affected your percent yield of product? 2. If the product was not completely dry when it was removed from the oven and massed, how would your percent yield be affected?

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