Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Experiment 9 β Decomposition of Baking Soda Introduction The decomposition of any metal hydrogen carbonate will result in the formation of that metal carbonate, water vapor, and carbon dioxide. This lab will investigate the decomposition of sodium hydrogen carbonate, NaHCO3, commonly called baking soda. It is governed by the following chemical equation: ο 2 πππ»πΆπ3 (π ) β ππ2 πΆπ3 (π ) + π»2 π (π) + πΆπ2 (π) In this lab, the NaHCO3 will be decomposed to form Na2CO3. The first half of the experiment will use a pure sample of NaHCO3, and the second part will use a mixture containing only some NaHCO3. Depending on how well the first half of the experiment is performed, the percent yield of this reaction should be 100%. The second half of the experiment should see something less than 100%, since it is not a pure sample. Percent yield is defined as the actual yield divided by the theoretical yield. The actual yield is the amount of product one produces during the experiment. The theoretical yield is the amount of product one should have produced. % πππππ = πππ‘π’ππ π¦ππππ π₯ 100% π‘βπππππ‘ππππ π¦ππππ Unless there are experimental errors, the % Yield should not be over 100%. In many cases, especially ones in which the reaction is difficult to control; the % Yield can be much lower than 100%. In the case of impure samples, like in the second half of this lab, the % Yield can give the percent of active material in the sample. The concept is the same. The only trick is to keep track of which value is the actual amount, and which is the theoretical amount. Refer to the sample calculations for more insight. Equipment 100 or 250 mL beaker Watchglass Bunsen burner Wire gauze Iron ring NaHCO3 Unknown sample Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Examples Percent Yield from a Pure Sample A 0.685 g sample of Ca(HCO3)2 is decomposed by heating to form CaCO3. When the sample is cooled it weighs 0.418 g. Calculate the theoretical yield of CaCO3. ο πΆπ(π»πΆπ3 )2 (π ) β πΆππΆπ3 (π ) + π»2 π (π) + πΆπ2 (π) To find the theoretical yield, calculate the amount of CaCO3 one expects to obtain from the Ca(HCO3)2. Set up the mass-mole-mole-mass calculation is as follows: 0.685 π πΆπ(π»πΆπ3 )2 [ 1 πππ πΆπ(π»πΆπ3 )2 162.12 π πΆπ(π»πΆπ3 )2 ][ 1 πππ πΆππΆπ3 1 πππ πΆπ(π»πΆπ3 )2 ][ 100.19 π πΆππΆπ3 1 πππ πΆππΆπ3 ] = 0.422907 π πΆππΆπ3 in proper sig figs = π. πππ π πͺππͺπΆπ Theoretical Yield The percent yield is actual yield divided by the theoretical yield. After decomposition, the actual yield of CaCO3 was 0.418 g. Divide this actual yield by the theoretical yield and convert to percentage. % πΆππΆπ3 = πππ‘π’ππ π¦ππππ π‘βπππππ‘ππππ π¦ππππ π₯ 100% % πΆππΆπ3 = 0.418π 0.423π π₯ 100% % πͺππͺπΆπ = ππ. π% Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Percent of Active Material in Unknown Mixture A 1.425 g sample of powder contains KHCO3 and an unknown amount of unreactive salt. When it is decomposed it weighed 1.225 g. Calculate the %KHCO3 in the unknown powder. ο 2 πΎπ»πΆπ3 (π ) β πΎ2 πΆπ3 (π ) + π»2 πΆπ3 (π) The only mass that is truly known in this problem is the mass that was lost, namely the H2O and CO2 (grouped as H2CO3 (g) in the equation above). The mass of gas that was produced is merely the mass before heating minus the mass after heating: 1.425 g β 1.225 g = 0.200 g gas released. Using this mass, we can determine the actual amount of KHCO3 that produced this gas by the following mass-mole-mole-mass calculation: 0.200 π π»2 πΆπ3 [ 1 πππ π»2 πΆπ3 60.03 π π»2 πΆπ3 ][ 2 πππ πΎπ»πΆπ3 1 πππ π»2 πΆπ3 ][ 100.02 π πΎπ»πΆπ3 1 πππ πΎπ»πΆπ3 ] = 0.666467 π πΎπ»πΆπ3 In proper sig figs = π. πππ π π²π―πͺπΆπ Actual amount of KHCO3 in sample The original sample was 1.425 g. If all of the sampler were KHCO3, then the amount of gas released would be much more (0.428 g, using the same calculation as in the first example). In this case we see that our calculated value above is the actual amount of KHCO3 that made the gas, and our initial measured amount in the problem statement is the theoretical amount that should have made the gas. The percent of active material is the amount of KHCO3 that decomposed (made the gas) divided by the total amount of material. % πΎπ»πΆπ3 = ππππ’ππ‘ ππππππππ ππ π‘ππ‘ππ ππππ’ππ‘ ππ πππ‘πππππ % πΎπ»πΆπ3 = π₯ 100% 0.666π 1.425π π₯ 100% % π²π―πͺπΆπ = ππ. π% Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Procedure Percent Yield of Na2CO3 from pure NaHCO3 Weigh out approximately 1 g of NaHCO3 into your beaker. Place the watchglass on the beaker. This will act as the indicator of when the experiment is done. Place the beaker on the wire gauze and heat the NaHCO3. A vigorous flame will be used in this experiment. Be sure that the tip of the cone of the flame is touching the wire gauze. Water droplets should begin to form on the watchglass within a few minutes. Note: the initial βfogβ one sees upon heating is merely the water vapor that is naturally occurring on the glass being driven off by the heat. Continue to heat the NaHCO3 until the water droplets on the watchglass have disappeared. Allow the beaker to cool to room temperature and weigh it. This is the mass of the Na2CO3 resulting from the decomposition. Calculate the theoretical yield of the Na2CO3 from the original mass of the NaHCO3. Percent of NaHCO3 in an Unknown Mixture Weigh out approximately 1 to 2 g of the unknown sample into your beaker. Repeat the steps of the previous procedure. This time, however, the resulting mass is the expected Na2CO3 and the unreactive component of the unknown. Use the mass lost in the experiment, the H2CO3 gas, to determine the mass of NaHCO3 in your unknown sample. Post Experiment Instructions All waste can be placed in the trash or rinsed down the sink. Return unknown vial to its place of origin. Figure 9.1: Setup of Decomposition Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Prelaboratory Questions A 0.685 g sample of NaHCO3 is decomposed by heating to form Na2CO3. When the sample is cooled it weighs 0.418 g. Calculate the theoretical yield of Na2CO3. Calculate the percent yield of Na2CO3. ο 2 πππ»πΆπ3 (π ) β ππ2 πΆπ3 (π ) + π»2 πΆπ3 (π) A 4.076 g sample of powder contains LiHCO3 and an unknown amount of unreactive salt. When it is decomposed it weighed 1.496 g. Calculate the %LiHCO3 in the unknown powder. ο 2 πΏππ»πΆπ3 (π ) β πΏπ2 πΆπ3 (π ) + π»2 πΆπ3 (π) Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Data Table Percent Yield of Na2CO3 from a pure sample of NaHCO3 Trial 1 Trial 2 Mass of NaHCO3 (before heating) __________ __________ Mass of Na2CO3 (after heating) __________ __________ Theoretical Yield of Na2CO3 β show calculations Theoretical Yield of Na2CO3 __________ __________ __________ __________ Percent Yield of Na2CO3 β show calculations Percent Yield of Na2CO3 Average percent yield of Na2CO3 __________ Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Percent of NaHCO3 in an Unknown Mixture Unknown Number ____ Trial 1 Trial 2 Mass of Unknown Mixture Before Heating __________ __________ Mass of Unknown Mixture After Heating __________ __________ Mass of gas released, H2CO3 __________ __________ Mass of NaHCO3 in Unknown Mixture β show calculations Mass of NaHCO3 in Unknown Mixture __________ __________ Percent of NaHCO3 in Unknown Mixture β show calculations Percent of NaHCO3 in Unknown Mixture __________ Average Percent of NaHCO3 in Unknown Mixture __________ __________ Experiment 9 β Baking Soda Name __________________ Lab Section __________________ Postlaboratory Questions 1) A 1.897 g sample of Mg(HCO3)2 was heated and decomposed. When the sample cooled, it weighed 1.071 g. What is the % Yield of this reaction? 2) An geologist brought in a sample of ore containing Fe(HCO3)3 and some other, unreactive material. If the sample weighed 1.394 g before it was decomposed, and 0.978 g after, what is the %Fe(HCO3)3 of the sample? Experiment 9 β Baking Soda Name __________________ Lab Section __________________ 3) A 1.112 g strip of magnesium is dropped into a giant container of HCl (aq). What is the volume of the gas that is produced at 0°C and 1 atm? 4) Calcium metal is dropped into water, and 15.7 L of gas is produced at STP, what was the mass of calcium that produced the gas? 5) If 2.112 g of Fe2O3 reacts with 0.687 g of Al, how much pure Fe will be produced? ο πΉπ2 π3 (π ) + 2 π΄π (π ) β π΄π2 π3 (π ) + 2 πΉπ(π )
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