A weak acid (HA) after treatment with 12 mL of 0.1 M strong base (BOH) has a pH of 5. At the end point, the volume of same base required is 27 mL. Ka of acid is (log 2 = 0.3)

mmoles of HA taken = 27 x 0.1 = 2.7                HA    + OH-       →        A-       +        H2O t=0         2.7         1.2 teq           1.5            -                       1.2 pH = pKa + log   A-HA  ⇒  5 =   pKa + log  1.21.5 5= pKa + log  45 ∴     pKa = 5.1                     ⇒  Ka = 8 × 10-6.