Sarah, I know April 5 was only yesterday BUT I have no recollection of that problem. I must have done 200 problems since then (well, perhaps not 200, perhaps more like 100) but after less than 24 hours all those problem run together. In the meantime I've gone to church, worked some puzzles, and gone out to eat Sunday lunch with friends. In general, however, one way to find the Ka of a weak acid is to titrate it with a strong base, then take 1/2 the volume required for exact neutralization, and the pH at that point will be pKa. The primary reason those values may not agree with published values is two fold.
1. It is extremely difficult to determine exactly where the equivalence point is FROM A GRAPH. The mid-point of that vertical portion of the pH vs mL curve is elusive and difficult to pin down to say 25.52 mL. The graph just doesn't lend itself to that kind of accuracy. So that is one source of error (and the largest). Another is that you aren't making a correctly for the ionic strength of the solution when you titrate it. Most of the Ka and Kb values we see in tables in texts are activity constants and not concn constants. I hope this answers your question. If not give me a hot link to the posted problem and I can try to refresh my memory.
Hi DrBob222,
this is regarding my Determining Ka of Acetic acid lab I talked about on April 5, 2008.
Refer to the volume of NaOH on your graph from question 2. Calculate half this volume and on your graph, find the pH when the solution was half neutralized.
did you get a pH in the range of 4.8 - 5.0 as well?
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You pretty much answered my question thanks! :)
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