Worked out solutions for the packet sent out on Friday - Cr(OH)3 problem

[robert zellmer]

Someone asked about a question from the packet I sent out on Friday.
It's #3 in the section about pH & solubility.

On 7/14/2018 9:13 AM, robert zellmer wrote:
> 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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