Worked out solutions for the packet sent out on Friday

[robert zellmer]

I sent a packet of extra problems from another instructor covering ch 17
problems (17.4-17.6) on Friday.  Here are the solutions from them by that
instructor.  I haven't checked all of them. I've also included a few 
comments
below.

Under the pH & solubility section for #3 (the Cr(OH)3 problem) it states
a pH = 10.00.  In the solution it states a solution buffered at a pH of 
10.00.
Simply stating at a pH of 10 and buffered at a pH of 10 are different 
questions.

You might also want to look at table 17.1 in the 14th ed (I think it's 
the same #
in the other editions).  It lists some of the more common complexes 
you'll run
into (and the Kf values).  Note that NH3 and CN- are involved in a number of
these complexes.  The OH- anion is also involved in complex ion formation,
particularly in amphoteric hydroxides (the book lists Al^3+, Cr^3+, 
Zn^2+ and Sn^2+
as examples).   You do NOT have to memorize all these complexes and Kf
values.  You will be told in some way in the problem what complex forms and
the associated Kf.

There are many questions concerning whether something would be more soluble
in the presence of different substances.

         a)  several questions involve complex-ion formation.  We 
discussed this in lecture
             in chapter 17 and will do so again in ch 23.  There are 
some common ligands
you should know which form complexes with metal cations.  You'll see halides
             (F-, Cl-, Br-, I-), CN-, NH3, SCN-.  See tables 17.1 and 
23.4 in the book.

         b)  OH- acts like a ligand (Ch 17 & 23).  Think of amphoteric 
oxides and hydroxides.
These react with both H+ (acid) and OH- (base) to dissolve.  The most
             common cations whose oxides and hydroxides are amphoteric 
are Al^3+,
             Cr^3+, Zn^2+ and Sn^2+ (end of section 17.5). Al(OH)3 is 
pretty insoluble
             hydroxide.  It's soluble in acid because all hydroxides 
react with acid.  It also
             dissolves in a basic solution (as the OH- conc. inc.) 
because it forms the complex
             ion, Al(OH)4^-, which is soluble.

         c)  If the anion of the insoluble salt is the conj. base of a 
weak acid it will act
             as a weak base and the salt will be more soluble in acid.  
For instance,
             CaF2 is more soluble in an acidic soln. because the F- ion 
reacts with a
             strong acid to form HF and the CaF2 becomes more soluble.

                 CaF2(s)  <==>  Ca^2+(aq)  +  2 F-(aq)

             The added H+ reacts with F- to form HF(aq),

                 H+  +  F-  <==>  HF

             This removes the F- from the top reaction causing it to 
shift to the right and
             more CaF2 dissolves.

             All insoluble hydroxides & oxides are more soluble in acid, 
as stated above.

         d)  Question #3 in the pH & solubility section.

             The question asks for the molar solubility of Cr(OH)3 at a 
pH = 10.00.  It
             doesn't state in a buffered soln with a pH of 10.00 like #1 
in this section.
             However, the solution given treats it as a buffered soln 
and thus the conc.
             of the OH- in the ICE table doesn't change.  You can see 
this in the ICE
             table in the solution.

             If this were done with a pH of 10.00 but the solution 
wasn't buffered you
             would need to put a "-x" in the Change line of the ICE 
table under the OH-
             and at equil you would have (1.0 x 10^-4 - x).  That would 
still be easily
             solved w/o having to ignore the "x".

Dr. Zellmer
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