Q EXPERIMENT 13: STOICHIOMETRY - SYNTHESIZING CHALK Introduction: In this experiment you will study a precipitation reaction between calcium chloride and sodium carbonate. You will collect, dry, and weigh the precipitate and compare this experimental yield to the theoretical yield you will calculate from the balanced equation. Background: A double displacement or metathesis reaction occurs when two soluble ionic compounds are mixed and form an insoluble ionic compound or a gas or a slightly ionized compound. We will study a reaction in which a precipitate is formed. The general scheme for such a reaction is AB + CD ? CB + AD, where A and C are cations and B and D are anions. For this experiment we will precipitate calcium carbonate from the reaction between sodium carbonate and calcium chloride. The reaction is: Na2CO3 (aq) + CaCl2(aq) ? CaCO3 (s) + 2 NaCl (aq) We will use approximately 0.02 mole of each reactant and expect to obtain approximately 0.02 mole of solid product, since the stoichiometric coefficients are all 1 in the balanced equation. You will need to calculate the limiting reactant, and the theoretical yield, from your measured amount of each reactant. For example, if we use 2.00 g CaCl2 x 1 mole = 0.0180 mole CaCl2 111 g CaCl2 and mix it with 2.00 g Na2CO3 x 1 mole = 0.0189 mole Na2CO3 106 g The maximum amount of CaCO3 we can expect is 0.0180 mole x 100 g/mole = 1.80 g The 1.80 g is the theoretical (calculated) yield of CaCO3 in this example. Your values may differ. We will compare the actual yield (experimental value) with the theoretical yield to find the % yield. . % yield = actual yield of precipitate x 100 theoretical yield Materials Needed Equipment Chemicals Clean dry beakers (2), Stirring rod Vacuum filtration apparatus & Buchner funnel Wash bottle Watch glasses (2) spatula Sodium carbonate (Na2CO3 Calcium chloride (CaCl2) #442 filter paper Procedure 1. Into two clean, weighed beakers weigh out separately, about 2 g of sodium carbonate and about 2 g of calcium chloride. Use the larger of the two beakers for the Na2CO3 (Weigh the samples to two decimal places; 3 sig figs) on a pan balance. 2. Add about 10 mL of distilled water to the CaCl2, and 20 mL to the Na2CO3 and stir each sample until dissolved. 3. Slowly, with stirring, add small portions of the Na2CO3 solution to the CaCl2. The mixture will begin to look frothy. Stir until the mixture vigorously until it is no longer frothy, and it settles to the bottom (about 5 minutes). 4. Weigh a piece of filter paper that fits the Buchner funnel (#42, 9cm diameter). Also weigh a watch glass. 1. We will use a vacuum filtration apparatus to obtain the precipitate. Your instructor will demonstrate its use. On each bench there is an apparatus for your use. Obtain a Buchner funnel from the stockroom. Place the funnel securely in the side-arm flask. Place the weighed filter paper into the funnel. Turn on the water attached to the vacuum system. Direct a stream of water from a wash bottle onto the filter paper. Make sure the filter paper is tightly held in the funnel and that suction exists. (Check by covering the funnel with your hand). Pour the contents of your beaker into the funnel. You must do this slowly or some of the precipitate will go through the filter paper. Try to get most of the precipitate out of the beaker by washing several times with small quantities of water and using a wash bottle filled with DI water to rinse the beaker. Don’t use more than 100 mL for the rinsings. 6. Shut off the water, remove the funnel, and remove the filter paper and sample. Use a spatula to pry the filter paper loose from the funnel. Place the filter paper and sample on a labeled, weighed watch glass. Let it dry in an oven for about ½ hour at 110oC. Weigh the sample with the filter paper and watch glass after cooling. Safety and Waste Disposal Name EXPERIMENT 13: REPORT STOICHIOMETRY Section _ Show all data and calculations in your lab notebook and attach the carbon copies to this cover sheet. DATA AND CALCULATIONS: The reactants: Weight of beaker with sodium carbonate Weight of empty beaker Weight of sodium carbonate Calculation of moles of Na2CO3 Show your calculations in your notebook Weight of beaker with calcium chloride Weight of empty beaker Weight calcium chloride Calculation of moles of CaCl2 Show your calculations in your notebook Use these calculations of moles of each reactant to determine the limiting reactant. The limiting reactant is The product Weight of filter paper with calcium carbonate Weight of filter paper Weight of calcium carbonate (This is the “actual” or experimental yield of product) Calculate the theoretical yield (in grams) of calcium carbonate from the moles of limiting reactant. Show your calculations in your notebook. g CaCO3 Calculate the % yield. Show your work in yur lab notebook. % yield QUESTIONS 1. If you had used 1.30 grams of barium chloride instead of calcium chloride, and 1.20 grams of the Na2CO3, what would be the theoretical yield of barium carbonate? (Write and balance the equation, determine the limiting reactant, and maximum yield.) 2. If 150.0 g of sulfuric acid and 150.0 grams of barium chloride are mixed, how many grams of sulfuric acid and how many grams of barium chloride remain after the double replacement reaction is complete? sulfuric acid barium chloride 3. Would there be any effect on the amount of precipitate in the experiment if tap water containing calcium ions is used instead of deionized water, to dissolve the reactants. Assume that Na2CO3 is present in excess. Explain your answer.

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