Buffers and pKa and pH

[gkangelexa]

An buffer solution consists of a mixture of a weak acid and its salt (conjugate base and a cation)
or a mixture of a weak base and its salt (conjugate acid and an anion).

When you titrate (for example) a weak acid with a strong base, then at the half equivalence point, there are equal amounts of the acid and its conjugate base, right?

i think i understand that... im confused where the pKa and the pH parts come in...

Why, when making a buffer solution, do we want to pick an acid whose pKa is close to the pH at which we want to buffer the solution?

[Mike H]

Buffers are generally figured to be most efficient at a pH within 1 unit from their pKa.

At the half-equivalence point, what is the pH? How does that compare to the pKa?

[Borek]

For a buffer to be efficient it needs to be able to absorb both H+ and OH- - that means it has to contain both weak acid and its conjugate base. If they are in a "reasonable" (comparable) quantities ratio of their concentrations is close to 1. If you put ratio of 1 into Henderson-Hasselbalch equation (http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch), what pH of the buffer solution will you get?