Question

What point in a titration curve corresponds to the pKa of a weak acid?

In a potentiometric pH titration of a weak acid, why does the pH change rapidly with the addition of NaOH near the endpoint but changes very slowly for pH values near the pKa of the acid?

Answer 1

What point in a titration curve corresponds to the pKa of a weak acid?

the point is essentially the halfpoint of the titration, i.e. the half volume when the titration is in equilibrium...

2)

it changes very slowly since it is a buffer, i.e. the equilibriums are shifted slower

HA <> H+ and A-

A- + H2O <--> HA

adding base produces more A- therefore, th epH change is slow

at the end, there is no acid left, so a very small concnetation/amount of base added will increase drastically the pH