How does pKa relate to acidity?
Consider the general reaction of an acid in water:
Now it is well known that for a given temperature, the extent of and acid base reaction is a constant:
Clearly, for strong acids,
Now this is a mathematical expression, which we are free to manipulate, provided that we do it to BOTH sides of the equation. Now we can take
But by definition
And thus, when we tritrate a weak acid with a base, and plot