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
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
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