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Lab 5 – Standardizing a NaOH Solution _ANAL 424_

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									                                     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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