Chemistry 151 Last Updated Dec. 2013 Lab 8: Precipitation Reactions and Limiting Reagents Introduction In this lab you will perform a simple precipitation reaction between strontium nitrate and potassium iodate. The resulting precipitate, strontium iodate, will actually be isolated as a hydrated compound, as shown in the equation below. You will also determine the actual, theoretical and percent yields of your reaction. Sr(NO 3) 2 + 2KIO 3 + H 2O Sr(IO 3) 2 H 2O + 2KNO 3 After isolating the precipitate by vacuum filtration, you will determine which reactant was the limiting reagent and which was present in excess. This will be done both quantitatively (i.e., based on your initial volumes and concentrations) as well as performing a simple qualitative test on the filtrate (the solution that passed through the filter paper). Procedure Note: You will perform parts II through IV twice for a total of two trials (see Data). In the interest of time, do both reactions simultaneously (e.g., you do one reaction while your partner does the other). Part I: Preparing your solutions 1. Using DI water, prepare a 150.0 ml aqueous solution containing 6.2 to 6.5 g of potassium iodate. 2. Using DI water, prepare an 120.0 ml aqueous solution containing 2.4 to 2.6 g of strontium nitrate. Part II: Performing the Reaction 1. Using a graduated cylinder, measure 50.0 ml of your potassium iodate solution then transfer it to a beaker 2. Rinse the graduated cylinder with a small amount (< 5 ml) of DI water, adding the rinse to the beaker, then measure 40.0 ml of strontium nitrate. 3. Mix the two reagents in the beaker and stir for five minutes. Leave the stir rod in the solution. Part III: Isolating the Product 1. Use a small piece of masking tape to label a watch glass with your and your partner s initials. Place a piece of filter paper in the watch glass and measure the combined mass of the watch glass and paper. Before making your measurement, first make sure your filter paper fits in the Büchner funnel you ll be using in the next step. 2. Assemble a vacuum filtration apparatus as follows: a. Clamp a filter flask to a ring stand b. Attach one end of a rubber hose to the side arm of the flask and the other to a vacuum line (yellow know) near your work area. c. Attach a Büchner funnel to the top of the flask. Place the filter paper you weighed in step 1 inside the funnel. d. Turn on the vacuum line and pour 5-10 ml of deionized water through the funnel. Not only does this test your set-up, but also wetting your paper allows a seal to form between the paper and the funnel. 3. If the precipitate has settled to the bottom of the beaker, stir the solution a few times to distribute the solid in the solution as evenly as possible. 4. Remove the stir rod from the beaker and rinse it with a few milliliters of deionized water from a rinse bottle. Allow the water to collect in the beaker with the reaction solution. 5. With the vacuum on, slowly pour the solution into the funnel. Try not to pour too much in at once, as it may cause the paper to detach from the funnel, losing precipitate in the process. 6. Hold the empty beaker over the funnel, upside-down and at an angle. Using the rinse bottle from step 4, rinse the beaker with a few milliliters of deionized water to remove any traces of precipitate, allowing the water to drain into the funnel.
7. Wash the precipitate with 3-5 ml of deionized water then allow the vacuum to run for about 30 more seconds. 8. Carefully remove the filter paper from the funnel and set it on the watch glass you weighed in step 1. 9. Place your sample in an area designated by your instructor to dry until the next lab period. Part IV Testing the Filtrate for Excess Reagent 1. Obtain two small beakers (50-100mL). Using a small piece of masking tape, label one as beaker #1 and the other as beaker #2 2. Pour 25 ml of filtrate into each beaker. 3. Add 10 ml of strontium nitrate to beaker #1 4. Add 10 ml of potassium iodate to beaker #2 5. Allow the two solutions to set for five minutes, recording any changes that occur. Waste Disposal Solution waste should be disposed of in the Inorganic Waste bottle. The precipitate and filter paper should be disposed of in the solid waste container (after weighing it the following lab period, of course)
Name: Section: Data Items marked with an asterisk will be completed the following week, after your sample has air dried. Everything else (including the post-lab) must be completed this week, before the end of the lab. Failure to do so may prevent you from completing next week s lab in a timely manner. 1) KIO 3 concentration, M 2) Sr(NO 3) 2 concentration, M Show your work for calculating the concentrations of each solution KIO 3 Sr(NO 3) 2 Trial #1 Trial #2 3) Volume Sr(NO 3) 2, ml 4) Volume KIO 3, ml 5) Mass of watch glass and paper, g *6) Mass of watch glass, paper and Sr(IO 3) H 2O, g *7) Mass of Sr(IO 3) H 2O, g 8) Using the initial concentrations and volumes of each reactant, determine which is limiting. Show your calculations below (if you used the same volumes each time, you only need to do one set of calculations).
9) Record your observations from Part IV. Beaker #1 Trial 1: Trial 2: Beaker #2 Trial 1: Trial 2: 10) Does your observations support your calculations on which reagent is limiting? Explain. 11) Theoretical yield of Sr(IO 3) 2 H 2O, g *12) Percent yield, % Average Show your work for calculating the theoretical and percent yields of trial #1 in the space below.
Name: Section: Post-Lab Questions 1. Why was it necessary to isolate the final product in this lab by filtration instead of simply decanting the solution, as you d done in previous labs? 2. Why was it better to allow the precipitate to air dry instead of heating it to dryness, as you d done in previous labs? 3. Explain how each of the following errors affect your percent yield (increase, decrease, no effect)? a) There was an error in the Introduction, and the precipitate actually formed as a dihydrate. b) In Part III, you fill the Büchner funnel with solution, causing part of the filter paper to lift off the bottom of the funnel. b) In Part IV, both beakers produced precipitate.
Name: Section: Pre-Lab Questions 1. Define the following. a) Precipitate b) Hydrated compound c) Theoretical yield d) Percent yield 2. Rewrite the equation from the introduction, adding phases to each reactant and product. Write the net ionic equation for this reaction. 3. Calcium bromide and sodium sulfate react as follows CaBr 2(aq) + Na 2SO 4(aq) CaSO 4(s) + 2NaBr(aq) If 30.0 ml of 0.30 M CaBr 2 is mixed with 40.0mL of 0.25 M Na 2SO 4 a) Which reactant is the limiting reagent? b) What is the theoretical yield (in grams) of the precipitate? 4. Watch the following video on properly using a Büchner funnel: http://youtu.be/1jygpcd8bke Note that in our lab we will be attaching our hose to a vacuum line instead of an aspirator. a) What s written on her shirt? b) How do you know if your vacuum is working?