Question

Weak acids are good buffers, but only at pHs close to their pKa value. Why is that?

Answer 1

for weak acids

pH = pKa + log [conjugate base] / [weak acid].

if [conjugate base] / [weak acid] = <1 always. then only it can act as effective buffer.

if we add base to the solution it reacts with weak acid forms corresponding conjugate base. so less increase in OH^- concentration.

if we add acid it reacts with conjugate base forms weak acid. so less increase in H⁺ concentration.

always [conjugate base] / [weak acid] < 1 only.

so pH always close to pKa of weak acid.