CHEMISTRY 31

Summer, 2016 - Dixon

Homework Set 2 (for Exam 2)

Chapter Problems Date to finish    

Set 2.1                                                                                                          June 23

Ch. 6 8, 15-17, 19, 21, 23, 25, 33, 36, 43, 48

Additional Problem 2.1 (4 points)

It is desired to determine the cyanide (CN^-) concentration of a solution that also contains CO₃^2-.  The method being used to measure CN^- also responds to CO₃^2-, so it is critical to at least partially separate the ions so that CO₃^2- is removed from CN^-.  A test solution, which is expected to be like actual samples, contains CN^- and CO₃^2- at concentrations 2.0 x 10^-5 M and 8.5 x 10^-4 M, respectively.  The goal in the separation is to be able to retain at least 99% of the CN^- in the original solution while decreasing the concentration of CO₃^2- to no more than 10% of the CN^- concentration.  Using K_sp values listed in Appendix F of the text for Ag containing solids, determine if this could be done.  Which anion would be precipitated first upon addition of Ag⁺? How much of that anion would be left at the Ag⁺ concentration where the other anion would start precipitating?  Does this allow compliance with the requirements listed above?  Assume that you could re-dissolve a precipitated anion by adding acid.  Ignore activity for this problem.

Solutions to Additional Problem

Set 2.2                                                                                                          June 30

Ch. 7                                              1, 3, 4a,b,  9, 12, 19, 21, 23, 27, 28a, b

Ch. 8                                              3

Additional Problem 2.2 (5 points)

SrCO₃ is a sparingly soluble salt.  Using the K_sp from Appendix F,

a) Determine the concentrations Sr²⁺ and CO₃^2- for a saturated SrCO₃ solution in pure water ignoring secondary reactions of Sr²⁺ and CO₃^2- and ignoring the activity from dissolved Sr²⁺ and CO₃^2-.

b) Using the concentrations of  Sr²⁺ and CO₃^2- found in a), determine the ionic strength of the solution, the activity coefficients of Sr²⁺ and CO₃^2-, and account for these in a recalculated concentration of  Sr²⁺ and CO₃^2- (only one iteration of this calculated is expected).  Continue to ignore reactions of Sr²⁺ and CO₃^2-.

c) Now, using equilibrium constant values from Appendices G, I and J, write out other significant reactions (e.g.of Sr²⁺ or CO₃^2- with water or with each other) needed to solve this problem using the systematic method.  For reactions from Appendix I and J with b or K values less than 10 (logb or logK values less than 1), you can ignore those reactions.  Additionally, write out a charge balance equation.

Solutions to Additional Problem

Set 2.3                                                                                                          July 7

Ch. 17                                            1, 3, 4, 11, 16

Ch. 22                                            7a

No Additional Problem