Question

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

Answer 1

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