Once you have calculated your Ka value, use the data to find the concentration of the unknown acids.
Vol. of acid solution=25mL
concentration of NaOH used = 1M
Buffer (pH) = 8.35 at 11.5mL
1/2 11.5=5.75mL which is a pH of 3.58
I did pH=pKa + log(acid/base) to find the pKa
pH=pKa=10^-3.58
Ka=2.63x10^-4
I don't understand what you've done but the HH equation should be pH = pKa + log (base)/(acid).
After you know the Ka value, assuming it is monoprotic, then
mLacid x Macid = mLbase x Mbase
To find the concentration of the unknown acid using the Ka value, you can use the Henderson-Hasselbalch equation:
pH = pKa + log([acid]/[base])
Here, [acid] refers to the concentration of the acid and [base] refers to the concentration of the conjugate base of the acid. Since you know the Ka value (2.63x10^-4) and the pH (8.35), you can rearrange the Henderson-Hasselbalch equation to solve for [acid]:
[acid] = 10^(pH - pKa)
Plugging in the values:
[acid] = 10^(8.35 - (-3.58))
[acid] = 10^(11.93)
[acid] = 1.59 x 10^11
Therefore, the concentration of the unknown acid is approximately 1.59 x 10^11 M.