Why Does pH Equal pKa Plus Log(Salt/Acid) in Titration?

[semc]

During my lab experiment, i am required to titrate acetic acid with sodium hydroxide and i am required to calculate the pKa from the titration curve. I want to ask why is pH=pKa + log(salt/acid) (the pH value is the midpoint of the neutralisation and salt/acid is the concentration of salt over concentration of acid.

Secondly i am required to comment on the strength of acid base on their pKa value. Is there any other factor that affect the strength of acid other then the electronegativity on the oxygen ion causing the ion to be stable/unstable? Am i correct to say that the chlorine on chloroacetic acid causes the electron on the oxygen ion to be MORE delocalised?

 

[Fightfish]

$$K_{a}=\frac{[salt][H^{+}]}{[acid]}$$
$$[H^{+}]=K_{a}\frac{[acid]}{[salt]}$$
Then simply take log on both sides, and multiply throughout by -1 to get the 'p'

 

[Borek]

Buffers and Henderson-Hasselbalch equation.