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<p><font face="serif">I have gotten several questions about how to
find pKa for your weak acids from lab last week. Please refer
to slide number 29 from the Chapter 17 notes. There you will
find the plot of a weak acid being titrated by a strong base.
In your lab you took solution of your unknown (a weak acid) and
split it in two. You then titrated half of the solution to the
endpoint. Look at the plot -- you took that solution to the
endpoint. You then combined the part of the solution that you
titrated to the endpoint with the other half that you had not
titrated. By recombining those two you moved back down the
titration curve to the point half-way to the equilvalence
point. At that volume that is halfway to the equivalence
point the pH=pKa. You measured the pH at that point -- so you
measured the pKa.</font></p>
<p><font face="serif">To find the Ka you just do the standard Ka =
10^{-pKa}</font></p>
<p><font face="serif"> </font></p>
<div class="moz-signature">-- <br>
<b><i>Andrew J. Pounds, Ph.D.</i></b><br>
<i>Professor of Chemistry and Computer Science</i><br>
<i>Director of the Computational Science Program </i><br>
<i>Mercer University, Macon, GA 31207 (478) 301-5627</i></div>
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