14.4Relative Strengths of Acids and Bases
Section · module m68805
What this needs
Learn these first — this section will not make sense without them.
- Alongside1.3Exponents and Scientific Notation
Ka = Kw/Kb is the same reciprocal-relation move used for reversed equilibria in chapter 13 (multiplying summed reactions multiplies their K values), here applied to conjugate acid-base pairs to compute Ka for HNO2 directly from Kb for NO2⁻.
Both Ka = [H3O+][A⁻]/[HA] and percent ionization = [H3O+]eq/[HA]0 × 100 are rational expressions in the equilibrium concentrations, and reading which factor changes with concentration (percent ionization is not constant like Ka) requires treating them as such rather than as fixed numbers.
- Alongside6.6Logarithmic Properties
The text derives Ka·Kb = Kw for a conjugate pair by multiplying the two K expressions; taking −log of both sides turns that product into pKa + pKb = 14 — the identity is only memorable rather than memorized once the log-of-a-product rule is in hand, and pKa notation is used later in the same chapter (buffers, indicators) assuming this connection.
Computing percent ionization from a given pH first requires converting pH to [H3O+] = 10^(−pH) — the same exponential-equation move as pOH↔pH conversion, chained into a rational-expression calculation.
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.