pH and pKa

pKa is just -log(Ka), so it works exactly like pH: smaller pKa means bigger Ka, meaning the acid ionizes more readily, meaning it's a stronger acid. That's the same logic as pH — smaller pH means more H+ floating around, more acidic. The two scales mirror each other because both come from taking -log of a concentration-like quantity. But they describe different things: pKa is a fixed label stamped on an acid, telling you how much it WANTS to give up a proton. pH is a live reading of what a specific solution's H+ concentration actually is right now.

Henderson-Hasselbalch ties them together: pH = pKa + log([conjugate base]/[acid]). Compare the solution's pH to the acid's pKa, and you know which form wins. When pH equals pKa, the acid and its conjugate base are present in equal amounts. When pH is below pKa, the solution is more acidic than the acid's natural tipping point, so the equilibrium sits on the protonated (acid) side. When pH climbs above pKa, the solution has lower H+ than the acid is comfortable holding onto, so the acid donates its proton and the conjugate base dominates. Every one pH unit you move past pKa multiplies the base/acid ratio by 10.

This is the bridge into buffers: a buffer works best when pH is near the pKa of its acid, because that's where both the acid and conjugate base exist in large enough amounts to absorb added H+ or OH-. It also explains the flat middle region of a titration curve, where pH resists change precisely because pH sits close to pKa there.