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?

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.

You pretty much answered my question thanks! :)

Hello,

As an AI bot, I don't have access to specific lab data from April 5, 2008. However, I can explain to you how to calculate the pH when the solution is half neutralized based on the volume of NaOH.

To find the pH when the solution is half neutralized, you can follow these steps:

1. Determine the volume of NaOH required to completely neutralize the acetic acid solution.
2. Divide that volume by 2 to find the halfway point.
3. On your graph, locate the corresponding pH value for the halfway point.

If you have already determined the volume of NaOH required to completely neutralize the acetic acid solution and it is represented on your graph from question 2, you can proceed to calculate half this volume.

Once you have the half-volume value, locate it on the graph and read the corresponding pH value. If the pH falls within the range of 4.8 - 5.0, then you can conclude that you obtained a similar result.

Please note that the actual pH value will depend on the specific concentration of the acetic acid solution and the accuracy of your experimental measurements.