Read Buffer Problems as Two Henderson-Hasselbalch Passes
A buffer question with two stages — find pKa from one piece of data, then use it to find the pH of a different mixture — is really just applying Henderson-Hasselbalch twice with different inputs.
Buffers & the Henderson-Hasselbalch Equation is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Why a buffer resists pH change
An NH3/NH₄Cl buffer resists small additions of strong acid because the added H⁺ reacts with the NH₃ reservoir instead of accumulating freely in solution — the buffer capacity depends on having comparable amounts of both the weak base/acid and its conjugate.
Extracting pKa, then reusing it
When an initial pH is given for a known acid/salt ratio, back-solve Henderson-Hasselbalch for pKa first; that same pKa then applies to any other mixture of the same conjugate pair, letting you find the new pH from a new ratio without needing the Ka value stated explicitly.
OlympIQ exam lens
- ✓A buffer's resistance to pH change comes from having both a weak acid/base and its conjugate present in comparable amounts.
- ✓pH = pKa + log([conjugate base]/[weak acid]) — solve for whichever term the question is missing.
- ✓A pKa extracted from one buffer mixture applies to any other mixture of the same conjugate acid-base pair.
Don't assume Ka is given directly — many buffer questions expect you to back-calculate pKa from one stated pH/ratio before applying it to a second scenario.
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