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EXPERIMENT 9 CHEMICAL REACTIONS: STOICHIOMETRY LABORATORY REPORT Your Name TA's Name You must read pages 69 and 70 to complete the following calculations. Lab Section 1411. Date Trial #1 Trial #2 Trial #3 vol . 0.100 M K2CO, solution -1200 mL 12.00 mL mmole K2CrO vol. 0.100 M Pb(NO,), solution 10.00ml mmole Pb(NO) 12.00m 14.00 mL 20 g mass product+paper 0.579 g0.654 g mass dry filter paper 0266 mass PbCrO, collected precip. 9 mmole Pbcro, CALCULATIONS AND QUESTIONS (USE THE SPACE ON THE BACK OF THIS SHEET IF NECESSARY) (20% ned by 1. In each trial determine which reagent was the limiting reagent? Give a reason, the results of your experiment, which proves your conclusion. 20% 2.Calculate the number of millimoles of K, Cro, in 10.55 ml of a 0.120 M solution. Report your answer with 3 significant figures (SF). 3.How many milliliters of a lead(I) nitrate solution (that is 0.1854 M in concentration) is ne sary to stoichiometrically react with all of the potassium chromate in 45.8 milliliters of a 56 M K,CrO, solution? (Work on the back of this page.) Report your answer with 3 SF. 4. If a titration using these solutions started with 25.00 mL of potassium chromate solution (05056 M), what volume of 0.2456 M lead(lI) nitrate solution would be required to reach the endpoint? What MASS of the products would be created in that reaction? (Work on the back of this page.) Report your answer with 4 SF 71 Laboratory Number 9Chemistry 1411 EXPERIMENT 9 CHEMICAL REACTIONS: STOICHIOMETRY EQUIPMENT None CHEMICALS This is a dry lab; no chemicals are needed This lab's procedures are not carried out because of the toxicity of the reagents involved Read PURPOSE The object of this experiment is to evaluate the nature of a double replacement reaction fully establishing the relationship between the balanced chemical equation and the reac- n conditions. No chemical procedures are carried out, but you must read and understand the by cedures involved as if you were performing the lab INTRODUCTION The reaction under study in this experiment only consists of two reactants, so the amount of one can be controlled while the amount of the other can be varied over a considerable range. The reaction is represented below. Pb(NO)2 (aq) + K,Cro, (aq)-> 2 KNO, (aq) + PbCrO.(s) In the experiment the volume of the 0.100 M potassium chromate solution will be maintained at 12.00 ml, while the volume of the 0.100 M lead(II) nitrate solution will be varied within the range 6.00 to 16.00 ml. Each pair of students in a lab section will be given three specific vol umes of lead(II) nitrate solution to evaluate. The entire lab will be able to set the reaction con ditions such that the initial amount of potassium chromate will always be 1.20 mmole for each trial, yet will test reactions with different concentrations of lead(II) nitrate. See "Useful Defini- tion" on the next page. When calculating millimoles of the reactants in the data table on page 70, use the data given in that table. If each student pair evaluates three possible concentrations, then within the lab section we can replicate and evaluate our experimental error Naturally, in any well designed experiment one must be able to measure something to determine the effect under investigation. Fortunately lead(lI) chromate is not a soluble salt, and will fall out of solution. Once it is determined that the reaction is complete, that is all of the lim iting reactant has been consumed, this solid can be filtered, dried and the mass determined. If all of the lab data is plotted, mass of precipitate vs. mmoles of Pb(NO,)2, then the class can determine the conditions under which the potassium chromate was the limiting reagent, and these two those under which the lead(II) nitrate was the limiting reagent. And, the intersection of areas on the graph indicates the exact relationship between the two reactants. A sample plot will be illustrated by your instructor on the black board This symbol means that very toxic chemicals are involved in the procedure described. Laboratory Number 9 Chemistry 1411 PROCEDURE Obtain your assigned volumes of Pb(NO), solution, two burettes, a wash bottle, several rounds of fine filter paper, and about 50.0 ml of each solution from your instructor. Wash and rinse three small beakers and the burettes. Using a few ml of the appropriate solution, wet the burette that you will be filling with that solution. Mark each beaker so that you can identify them as they will contain different volumes of the Pb(NO), solution. Using a burette, deliver 12.00 ml of 0.100 M K,CrO4 solution into each of the three clean, dry beakers. Using the other burette, and while you are swirling the solutions, slowly add the required amount of lead(II) nitrate solu- tion. At this point add 10.0 ml of 5% nitric acid to each. This is simply to assist in the formation of large crystals of leadqlI) chromate. With stirring, carefully heat each solution over a burner but DO NOT allow the solution to boil!!! At this time most of the precipitate will settle to the bottom of the beaker. Remember also that you want to be able to measure the exact amount of lead(II) chromate that formed during each reaction so DON'T mix with the same stirring rod. DON'T spatter product out of the beaker, and DON'"T spill anything!! Determine the mass of a piece of filter paper using the best balance in the lab. Fold the paper and place it in a funnel. Moisten the filter paper with distilled water and press the top of the paper to the funnel. Using the stirring rod decant the KNO,- HNO, solution into the funnel. Next wash the precipitate in the beaker with 10.0 ml of distilled water. Quantitatively transfer ALL of the precipitate into the filter. Use the rubber policeman and a small stream of water from the wash bottle to move every teeny, tiny bit of the solid into the filter paper. At this stage you should also avoid any other piece of junk that could get in there. It is supposed to be ONLY lead(lI) chromate, distilled water and filter paper. Carefully remove the filter paper and place it on a MARKED paper towel in your locker to be weighed next week During the next laboratory period determine the mass of the filter paper and lead(II) chromate. Subtract the appropriate mass for the filter paper and report this data to your instructor. Make sure that all of the wastes from this laboratory are disposed of properly. Useful Definition: A millimole is a word that describes a number of things similar to the mole. that is 2 molar lead nitrate contains 2 moles of Pb(NO,), per liter and 2 millimoles of lead nitrate A millimole is defined as 0.001 moles (and a millimeter is 0.001 meters). Therefore a solution SO.

Answer 1

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