When you have a situation like the one in this problem, why does the pH at first lower then when the concentration of CH3COO is higher than the CH3COOH it raises the pH? Calculate the pH of a solution formed by adding 0.20 moles of NaOH to 1.00 liter of 0.50 M acetic acid. Ka = 1.8 x 10^-5 for acetic acid.

I'm semi-brain dead right now. I have been studying for the past 2 weeks straight, so I apologize if the answer is obvious.

Question

When you have a situation like the one in this problem, why does the pH at first lower then when the concentration of CH3COO is higher than the CH3COOH it raises the pH? Calculate the pH of a solution formed by adding 0.20 moles of NaOH to 1.00 liter of 0.50 M acetic acid.    Ka = 1.8 x 10^-5 for acetic acid. 

I'm semi-brain dead right now. I have been studying for the past 2 weeks straight, so I apologize if the answer is obvious.