CHEM 2010

							     CHEM 2010                                                     Name _______________________
     Dr. Byrd                               Quiz 3
     11/29/07
Instructions: Pay very close attention to units and significant figures in doing the problems. Neatly
              presenting your work will be helpful to both of us.
              There are two parts to this exam. Part 1 has only one question which is from Chapter 16.
              Part 2 has three questions, one each from chapters 13, 14, and 15. Do only one of the
              problems in part 2, and put an “X” across the two you do not want graded.

      Part 1. 40 points.
      The standard method for determination of protein in food is the Kjeldahl method. In a typical
      analysis a 0.8025 g sample of a canned meat product was digested in hot, concentrated sulfuric acid
      to convert bound nitrogen (N) to ammonium ion. Concentrated sodium hydroxide was added and the
      resulting ammonia was distilled into 25.00 mL of 0.05000 M HCl. The excess HCl was then back-
      titrated with 2.16 mL of 0.04000 M NaOH.

      a) (35) Calculate the % N in the sample.


      b) (5) Protein in meat is determined by multiplying the % N by 6.25. Calculate the % protein in the
      sample.


      Part 2. 60 points. Do only one of the three.

1. (60) For the titration of 40.00 mL of 0.100 M oxalic acid (abbrev., H2Ox) with 0.200 M NaOH(aq)

      a) (12) write the relevant equilibria and equilibrium constant expressions
              (Note: K1 = 5.6 x 10-2, K2 = 5.42 x 10-5, Kw = 1.0 x 10-14)


      b) (24) calculate the pH at the first equivalence point, justifying any assumptions you make.


      c) (24) Derive an expression for  HOx1 and calculate  HOx1 at the first equivalence point.


2. (60) For the titration of 30.00 mL of 0.100 M HNO2(aq) with 0.150 M NaOH(aq) calculate the several
       points on the titration curve as outlined below.
       (Note Ka = 7.1 x 10-4)

      a) (5) Write Byrd’s magic equation below.


      b) (50) Calculate the pH of the resulting solution after addition of each of the volumes listed below.
      You must justify any assumptions you make.
             i) (10) 0.00 mL of 0.150 M NaOH


             ii) (10) 10.00 mL of 0.150 M NaOH


             iii) (10) 19.96 mL of 0.150 M NaOH


             iv) (10) 20.00 mL of 0.150 M NaOH


             v) 20.04 mL of 0.150 M NaOH


      c) (5) Use the Table of Indicators shown to choose an appropriate indicator for the titration. Explain
      your choice and describe the expected color changes.




3. (60) Calculate a small segment of the titration curve for the titration of 50.00 mL of 0.0400 M KBr(aq)
       with 0.0500M AgNO3(aq) as outlined below (Note: Ksp(AgBr) 5.0 x 10-13).

      a) (10) Write any relevant equilibria and equilibrium constant expressions.


      b) (10) Write the charge balance equation for the solution during the titration.


      c) (10) Rewrite the charge balance equation in terms of [Ag+] and known concentrations and volumes
      of the AgNO3 and KBr solutions. Rearrange to form a quadratic in [Ag+].


      d) (30) Calculate [Ag+] and pAg after the addition of 39.92, 39.96, 40.00, 40.04 and 40.08 mL of the
      AgNO3(aq) solution. Justify any assumptions you make.

             i) (6) 39.92 mL

             ii) (6) 39.96 mL

             iii) (6) 40.00 mL

             iv) (6) 40.04 mL

             v) (6) 40.08 mL