Re: Coordination Equilibrium

On 26 Nov 2005 09:21:00 -0800, "Paddan" <patrik@xxxxxxxxxxxxx> wrote:

>Calculate [Cu2+] in a solution with [Cu2+]-total = 0,1 M and [NH3] = 1
>M where lg k1 = 4,25, lg k2 = 3,61, lg k3 = 2,98, lg k4 = 2,24 and kp =
>13,08.
>
>How do you do it?


Psyche it out. Unless you are prepared to handle solving many
simultaneous equations, the usual approach is to use your chemical
intuition to guess the approximate final outcome. You then use those
"guesses" to solve for what you want; while doing that, you are alert
to see whether your guesses work well in the equations.

In this case, the ammonia is in large excess. Thus it is a good guess
that the copper ions are "fully" complexed with NH3. Thus you know the
concentration of the tetra-NH3 complex, and of the remaining free NH3.
You can now calculate the conc of other Cu species -- all of which
will be negligible compared to the tetra-NH3.


(It is like assuming that the amount of a weak acid that is ionized is
small compared to the total amount added. And you check this as you
try using the approximation.)


What is Kp for ??


bob

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