Calculate the pH at the following points during the titration of 100.0 mL of 0.20 M acetic acid (Ka for acetic acid = 1.8 x 10-5) with 0.10M sodium hydroxide
1. after addition of 100.0mL of base
2. after addition of 200.0mL of base
3. after addition of 250.0 mL of base