Buffers show up right after weak acid equilibria, and the good news is that they don't require new chemistry, just a new way of using the equilibrium ideas already covered.
What makes a buffer a buffer
A buffer is a solution containing a weak acid together with its conjugate base, or a weak base together with its conjugate acid, both present in meaningful amounts at the same time. That combination lets the solution neutralize small additions of either a strong acid or a strong base without a large pH swing.
The Henderson-Hasselbalch equation
The equation
Worked example
Notice that when the acid and conjugate base concentrations are equal, the log term becomes log(1), which is zero, and pH simply equals pKa. That's the exact same idea behind the half-equivalence point on a titration curve.
Buffer capacity has a limit
A buffer's capacity, how much strong acid or base it can absorb before its pH starts changing significantly, depends on the actual amounts of the weak acid and conjugate base present, not just their ratio. A buffer made from very dilute solutions runs out of capacity fast, even if the ratio is ideal.
Where students actually lose points here
Using Henderson-Hasselbalch on a non-buffer solution
It only works for an actual buffer mixture. Using it for a solution of just a strong acid, or past the equivalence point of a titration, gives a wrong answer that can still look reasonable.
Flipping the ratio in the log term
The equation uses conjugate base over acid, [A−]/[HA], not the reverse. Flipping it gives the wrong sign on the log term and a noticeably wrong pH.
Treating buffer capacity as unlimited
A buffer resists pH change only up to a point. Adding enough strong acid or base eventually uses up one of the two components entirely, and after that the pH changes rapidly just like an unbuffered solution.
A direct application of equilibrium
Buffers are one of the clearest examples in the whole course of a real, practical use for equilibrium concepts, which makes this an especially common free response topic.
Common questions
What is the Henderson-Hasselbalch equation used for?
It calculates the pH of a buffer solution directly from the acid's pKa and the ratio of conjugate base to acid concentration, without needing a full ICE table calculation.
Why does a buffer's pH equal pKa when the ratio is 1 to 1?
Because log(1) equals zero, so the Henderson-Hasselbalch equation reduces to pH = pKa exactly when the acid and conjugate base concentrations are equal.
Does a buffer resist pH change indefinitely?
No. A buffer has a finite capacity based on the actual amounts of weak acid and conjugate base present. Once enough strong acid or base is added to use up one of those components, the buffer stops working and pH changes rapidly.