How does a pKa value relate to pH?
Consider the general acid dissociation reaction in water:
Now, this is an equilibrium reaction, and for a given temperature, we can write the equlibrium expression:
This is an equation, and like any equation we can add/subtract/multiply it etc. and maintain the equality, SO LONG AS WE DO THE SAME OPERATION TO BOTH SIDES OF THE EQUATION. We can certainly take the
ON REARRANGEMENT, subtract
So, upon titration of a weak acid, at the point of half-equivalence when