Titration Curve, finding Dissociation constant?

Question

Hi, I had a lab today where we find constant using Titration curve, im not sure how to do that last part.

My average of volumes, for pH 7 = 6.97. I think I devide this by 2 to get the half equivalence point, which would be 3.49mL at that my pOH was 4.51,

I was titrating 10mL of unknown acid with .1000M NaOH

the equation I use is pOH = pKb + log [conjugate acid]/[base]

Im not quite sure what to do from there, I dont know how to get the conjugate acid, and what is the base .1000 because thats what its molarity is? Do I plug 4.51 into the pOH and then devide log conju acid/base by 4.51 to get pKb, is pKb the dissociation constant?

Any help or infomation would be appreciated.

Answer 1

You are going the wrong way.
The dissociation constant of a weak monoprotic acid is Ka. If you have a polyprotic acid then you have Ka1,Ka2,etc and more than one equivalence points.

Do the plot, find the equivalence point, and then pKa=pH for 1/2 volume of base of that needed to reach the equivalence point.
Since you have a weak acid you don't have the equivalence point at pH=7 but at higher pH (The equivalence point will be at the middle of the rising curve).
Then Ka=10^-pKa

You can have a look at

http://www.shodor.org/unchem/basic/ab/

http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html