[CHM 112] Finding pKa of your Unknown

[Andrew J. Pounds]

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.

To find the Ka you just do the standard Ka = 10^{-pKa}

-- 
*/Andrew J. Pounds, Ph.D./*
/Professor of Chemistry and Computer Science/
/Director of the Computational Science Program /
/Mercer University, Macon, GA 31207 (478) 301-5627/
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