pH of a mixed solution

First of all, the mole ratio between acetic acid and sodium acetate is 4:5 according to the given. And hence the concentration will be in the same ratio too.
Then, using the data pKa = 4.76 for acetic acid:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/Chem/Buffer15.jpg

Which is the pH value of the mixture immediately after mixing.
Now, when the NaOH is added, 0.1 x (5/1000) moles of acetic acid are removed to give 0.1 x (5/1000) moles of more sodium acetate.
So the mole ratio between sodium acetate and acetic acid now is 2:1 by calculation and hence:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/Chem/Buffer16.jpg

(Note: In both calculations above, an assumption that the introduction of further H+ ions due to the dissociation of acetic acid at the point concerned is negligible compared to those originally present in the solution.)