A 15.0--mL unknown sample of aqueous acetic acid, CH3COOH(aq), was titrated using 0.800 M NaOH. If the titration endpoint was found to be 20.0 mL, what is the molarity of acetic acid in the sample? Calculate the pH of the solution after adding 0.0, 5.0, 10.0, 20.0, and 21.0 mL of titrant.