Exactly 100 ml of 0.15 M acetic acid (HC2H3O2, Ka=1.8 x 10^-5) are titrated with a 0.20 M NaOH solution. Calculate the pH for

(a) the initial solution (HC2H3O2 )

(b) the point at which 37.5 ml of the base has been added.

(c) equivalence point 

(d) after adding 80.0 ml of 0.20 M NaOH.