Common-ion example 1 in notes

[robert zellmer]

Students often ask many questions about common-ion problems,
mainly how we can do them by treating them as if we add the substances
simultaneously to the beaker (when in reality that's not what we are
doing).  I've attached an explanation as to why we can do these problems
as I did in class.  I did the problem first as it would be done 
following what
is actually physically occurring (adding the common ion to an existing
equilibrium).  Next I did it assuming we add everything simultaneously to
the beaker.  You will see we get the same results but the 2nd way is a
whole lot easier.

Note by having the common ion (no matter how you handle it) not
as much reaction occurs between the NH3 and H2O and the rxn
doesn't go as far to the right as it did w/o the common ion.  This
means less OH- is produced and the solution with the common
ion (NH4^+) is less basic than it would be w/o the common ion.
This makes sense since NH4^+ is a weak acid.  The soln is less
basic (becomes more acidic when the weak acid is added).

Dr. Zellmer
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