A 0.1276 g sample of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.0633 M NaOH solution. The volume of base required to bring the solution to the equivalence point was 18.4 mL. After 10.0 mL of base had been added during the titration, the pH was determined to be 5.87 What is the Ka of the unknown acid?