CHEM 102 Spring 2010 Lab 3 – Qualitative Analysis Part I Introduction: We will be looking at the chemistry of the cations, Ag+, Pb2+, Ni2+, Ba2+, and Fe3+ – silver, lead(II), nickel(II), barium, and iron(III). (In the context of this lab, the lead, nickel, and iron will be referred to without their oxidation numbers, because these oxidation numbers will not be changing in this lab.) The basic idea of the lab is to figure out how to determine which of these ions is present in a solution which contains at least one of these ions. In the first week of the experiment, you will explore how each of these ions reacts under various conditions with various other reagents. Then you will devise a scheme to identify the presence or absence of each of the ions. In two weeks, you will be given a solution which contains 1, 2, 3, or 4 of these 5 ions. Your task will be to identify which ions are present in this unknown solution. Procedure: Obtain approximately 10 mL of 0.1 M solutions of AgNO3, Pb(NO3)2, Ni(NO3)2, Ba (NO3)2, and Fe(NO3)3 in separate, clean, labeled beakers. You are to subject each of these solutions to the six tests below and record your observations in the table provided. Be sure to record colors before and after and whether or not a precipitate forms. 1. Put 10 drops of the test solution into a small test tube. Add 5 drops of 3 M H2SO4. 2. Put 10 drops of the test solution into a clean small test tube. Add 5 drops of 6 M HCl. If a precipitate forms, mix the sample and centrifuge. Decant the liquid, add 10 drops of deionized water, stir, and centrifuge again (this is known as washing the precipitate). Wash the precipitate a second time. Save the precipitate for test 3. 3. Add 2 mL of deionized water to the precipitate from test 2 and mix. Heat the tube in a boiling water bath for 4 minutes, stirring frequently. Centrifuge and decant the hot liquid. To the hot liquid, add 1 drop of K2CrO4 solution. 4. Put 10 drops of the test solution into a clean small test tube. Add 10 drops of 6 M NaOH. If a precipitate forms, stir and centrifuge. Discard the supernatant (the liquid portion) and save the precipitate for test 5 or 6. 5. Wash the precipitate from test 4 once with deionized water. Add 10 drops of NH3 solution to the precipitate and stir for one minute. Centrifuge. 6. Repeat test 4 to obtain a fresh sample of the hydroxide precipitate. Wash the precipitate once with deionized water. Add 5 drops of 6 M HCl to the precipitate and stir. Add 2 drops of 1 M NH4SCN solution. CHEM 102 Spring 2010 Test and Reagent Ag+ Pb2+ Ni2+ Ba2+ Fe3+ 1. SO42- 2. Cl- 3.a. heat b. CrO42- 4. OH- 5. NH3 6. a. H+ b. SCN- CHEM 102 Spring 2010 Data Analysis: From your data, complete and balance the equations listed below. Some of your answers will be speculations. Use the following guidelines. (1) Assume that most reactions are a simple combination of cations and anions. (2) If the product is dissolved, it is ionic, and the ions are separated. (3) If the product is solid, it is neutral. (4) NH3 forms a complex ion with some metals, with a formula like M(NH3)2n+. If you observed no change when the particular reagents were mixed, write NR for “no reaction.” Ag+ + SO42- __________ Pb2+ + SO42- __________ Ni2+ + SO42- __________ Ba2+ + SO42- __________ Fe3+ + SO42- __________ Ag+ + Cl- __________ + heat __________ + CrO42- __________ Ba2+ + Cl- __________ + heat __________ + CrO42- __________ Ni2+ + Cl- __________ + heat __________ + CrO42- __________ Ba2+ + Cl- __________ + heat __________ + CrO42- __________ Fe3+ + Cl- __________ + heat __________ + CrO42- __________ Ag+ + OH- __________ + NH3 __________ Ag+ + OH- __________ + H+ __________ + SCN- __________ Pb2+ + OH- __________ + NH3 __________ Pb2+ + OH- __________ + H+ __________ + SCN- __________ Ni2+ + OH- __________ + NH3 __________ Ni2+ + OH- __________ + H+ __________ + SCN- __________ Ba2+ + OH- __________ + NH3 __________ Ba2+ + OH- __________ + H+ __________ + SCN- __________ Fe3+ + OH- __________ + NH3 __________ Fe3+ + OH- __________ + H+ __________ + SCN- __________ CHEM 102 Spring 2010 Post-lab Questions: 1. If you were given a solution and asked to determine whether or not Ba2+ was present in the solution, what sequence of tests would you use. What results would you expect if Ba2+ was present? What results would you expect if no Ba2+ was present? 2. What sequence of tests would you use to determine if either Ag+ or Pb2+ or both were present in an unknown solution? Describe what you would expect to observe if a. The solution contained both Ag+ and Pb2+. b. The solution contained Ag+ but not Pb2+. c. The solution contained Pb2+ but not Ag+. d. The solution contained neither Pb2+ nor Ag+. Prelab questions: 1. Write the balanced net ionic equations for the following reactions: a. AgNO3(aq) + HCl(aq) b. Pb(NO3)2(aq) + HCl(aq) 2. Use your textbook to predict which of the cations in this experiment will form an insoluble compound with SO42-.
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