I'm having trouble with a write up for my anal chem lab if anyone can help.
I used a pH meter to measure the equivalence points of unknown acids now I have to use the ka's to determine what the substances were, but I can't remember how.
Here's the info I have figured out so far if anyone can help

at the first equiv. pt. 18.5 mls 0.01M NaOH added pH=5.58
2nd equiv. pt. 37 mls 0.01M NaOH added pH=9.00
then i have to use the first and second equiv. pt. to determine the ka3
any help would be great

Thats my problem i always use H-H but I have to use the ph at equivelence pt

none at all? well thanks for your help. as for the third ka do you know how to get that from the first. sorry but my TA isn't returning my emails

I'm having trouble with a write up for my anal chem lab if anyone can help.
I used a pH meter to measure the equivalence points of unknown acids now I have to use the ka's to determine what the substances were, but I can't remember how.
Here's the info I have figured out so far if anyone can help

at the first equiv. pt. 18.5 mls 0.01M NaOH added pH=5.58
2nd equiv. pt. 37 mls 0.01M NaOH added pH=9.00
then i have to use the first and second equiv. pt. to determine the ka3
any help would be great

your TA is probably not answering your emails because you're asking something that's so obviously stupid; everything should be described perfectly in your lab manual.  I'm guessing that determining the third equivalence point would not be feasible by the titration curve, even the second equivalence curve is probably not very visible.  You should probably be using the ampholyte equation (ring a bell?) do estimate Ka3 assuming the Kas are 10^4 magnitudes apart.

Not always true, I have seen lots of awful lab manuals.no stupid questions in science, this is a poor reason to not answer a students question.I am probably asking a stupid question too, but how do you do this with the data provided?

at times it's simply the case that the student has not read the manual, yes teachers are inadequate at times.  

From what I remember, you can estimate the third pKa, assuming that Ka s are at least 10^4 magnitudes apart using one of the forms of the ampholyte equation, there are actually two ampholytes involved, for instance if you were to place the first pure ampholyte in an acqueous solution at an appropriate concentration you can simply use pH=.5(pKa1+pKa2) or the complex mass action form.  You can find the similar equation for the second ampholyte (or is it amphoteric?).  There are, I believe methods through pure calculation, however you can also do it by observing the titration curve and finding the pH at the third equivalence point (don't need to actually see the steep slope).

GCT: using approach you described you may as well select at random any number above 11 and assume it is pKa3. It has nothing to do with determination, it is just wild guessing.

under proper conditions, it's a good estimation.  In fact, you don't actually need the derived equation common sense will get you through.  You can place a pure phosphate solution with appropriate conditions (not too concentrated for instance) and if you know Ka1 and Ka2, you can theoretically calculate Ka3 simply from the observed equilibrium pH.

If you take 0.01M phosphoric acid its solution has a pH of about 2.27 (with activities taken into account). This value will not change regardless of whether pKa3 has a value of 7 or 22 . The same will happen in case of 0.0001M  and 0.000001M solutions. Third dissociation step is just too far to make any difference.

In case of hydrogen phosphate solutions situation looks slightly better, but small errors in pH measurements means large difference in the pKa3 value. So the estimation is still wrong.

I'll try solving the problem later using the mass action derivative, the ampholyte equation form is not the same from what I remember for the two ampholytes.

Browsing older threads I have found this one:Is it already later?

That's GCT standard method - 'I'll show you are wrong at some later point' - but he never does.

get a life Borek