# How does pKa relate to acidity?

##### 1 Answer

Consider the general reaction of an acid in water:

#### Explanation:

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

On rearrangement:

But by definition

And so

And thus, when we tritrate a weak acid with a base, and plot