Topic #1: pH Titration
Recommended problems in Campbell, Chapt. 2: #6-10.
Model reaction: HA <=>
A- + H+
Dissociation constant: Ka = [A-][H+]/[HA]
-
The Henderson-Hasselbalch equation is:
pH = pKa + log(A-/ HA)
,
where
-
pH = the measured pH, (-log[H+]);
-
pKa = -logKa;
-
HA = the concentration of the acid, and
-
A- = the concentration of the conjugate base.
The initial pH of 100 ml of a monoprotic acid is shown below. We want to
find 1) the pKa and 2) the concentration of the acid by titrating
it with 1.0 M NaOH.
This is done in three steps. (Each calculated pH value has a small "experimental
error" added to it.)
3. Record the volumes of NaOH and the pH's you obtain; then graph them
as pH vs. equivalents of NaOH. You should get enough data so as
to have 3 or 4 values of pH, both above and below the pKa value
and within ±1.0 of the pKa.
Note:
In contrast to a "wet lab" titration where the pH would increase stepwise
with each addition of NaOH, this titration uses a new solution of acid
for each volume of NaOH added.
(Hint: Graph the values you obtain as they are calculated; then
as the shape of the curve becomes apparent, choose values for NaOH volume
that fall into the appropriate range.)
Answers to this problem.
A sample Answer Sheet for a similar
problem shows the format of the results and the graph required. |
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to the pH Titration Problem assigned for Problem Set #1.
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