[Galiligp]

hello;
i had a lab experiment involving a strong base NaOH 0.1 M and a unknown weak acid. we took 25 ml of the weak acid and add to him 50 ml of pure water. then we started to insert 1 ml of NaOH each time and write down the pH level. we did it twice and in the second time we were more priciest. with the results i was asked to make 2 graphs. one is the pH level according to the volume of the NaOH added and the other was the delta pH/delta X vs X (x= volume of the added base). i did the graphs and asked to find the pKa with pH=pKa +logx/Vb-x
now i need your help plz:
in the second graph the steepest point tell me the volume of the base in the equivalence point - right?
then i need to use pH=pKa +logx/Vb-x to find out the pKa but i don't manege to use it. i know that in the half equivelnt point the ph=Pka but i cant get this figures with the formula
the blank pH of the weak acid was 3.26.

thank you all in advance

nir

[Galiligp]

correction: we took 25 ml of the weak acid solution. and not like i wrote (25 ml of the weak acid).

anyway i got the equivalence point after 9.5 ml of the strong base was added.
from this i understand that: 0.1 mol/literX0.0095 L = 9.5X10^-4 mol of strong base. in the equivalence point we have the same amount of acid and base so we've got the same number of mols, right?
now i divide 9.5X10^-4 mol / 0.075 L to get my Molarity for the weak acid = 0.012 M
is it right? and how do it use the pH=pKa+logx/Vb-x formula?

thank you
nir