HH eqn for buffers

[robert zellmer]

I've been asked the following before by other students:

**********************
Dr. Zellmer,

I was reading the book and saw that for the Henderson Hasselbach
equation, the concentrations of the acid and base equal to the
concentrations from the equilibrium but not the original concentrations.
is that true?

**********************

In reality there are two answers to this (which I addressed in
lecture):

Technically they are the equil. conc. When asked to calc. the ratio
of base/acid from the pH and pKa you are actually getting the ratio
of the equil. conc. of  base and acid.

However, in the normal practice of determining the pH of a buffer
solution we use the original starting conc. because we usually ignore
the "x", just as we would when using an ICE table.  If we didn't do
this there would be no point in using the eqn. and we would just use
an ICE table every time (which you can do).  We do it this way, using
the original conc. of the acid and base, when we calc. the pH of a soln.
from conc. and the pKa.

Dr. Zellmer
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