Question

Why is the `pH` of a buffer close to the `pK_a` of the acid used to make the buffer...?

Answer 1

The `pH` of a buffer is tolerably close to the `pK_a` of the acid because this gives the greatest buffer capacity.

Explanation:

Buffers are formed from a mixture of appreciable quantities of a weak acid and its conjugate base. The `pH` of the solution is tolerably close to the `pK_a` of the acid, which of course will be somewhat below `7`.

The buffer equation gives us:

`pH=pK_a + log_10([[A^-]]/[[HA]])`

Of course when `[A^-]=[HA]` (at the point of half equivalence in a titration), `pH=pK_a`, because `log_(10)1=0`.