14.6Polyprotic Acids
Section · module m68807
What this needs
Learn these first — this section will not make sense without them.
Each ionization step of a polyprotic acid is its own rational expression (Ka1 = [H3O+][HCO3⁻]/[H2CO3], Ka2 = [H3O+][CO3²⁻]/[HCO3⁻]), and the second step's calculation substitutes the first step's solved [H3O+] and [HCO3⁻] directly into the second rational expression — a chained rational-equation solve.
- Alongside2.6Quadratic Equations
Treating the two ionization steps separately is licensed by an explicit rule of thumb (Ka1 at least 20× larger than Ka2), which is itself an approximation criterion of the same family as the small-x rule — deciding when the exact algebra (a coupled system) can be skipped in favor of two easier separate solves.
What this touches
Algebra that turns up here. Not a blocker, but this is where you will see it used.
No algebra turns up here. Most of the descriptive sections need none.