3.1.10 (HL)—Acid/base strength constants

$Ka = [A−][H3O+] / [HA]; Kb = [BH+][OH−] / [B]$

Ka and Kb measure dissociation extent. pKa = −log Ka, so a lower pKa indicates a stronger acid; use the corresponding comparison for bases and pKb.

Large Ka and small pKa both indicate the stronger acid; large Kb and small pKb indicate the stronger base. Strength describes extent of ionization, whereas concentration describes amount per volume—dilute and weak are not synonyms.

Representative question

[Maximum number: 1]

State the $K_{\mathrm{a}}$ expression for ethanoic acid.

$K_{a}=\frac{\left[\mathrm{CH}_{3} \mathrm{COO}^{-}\right]\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]}{\left[\mathrm{CH}_{3} \mathrm{COOH}\right]}$

Marking guidance:

Accept $\mathrm{H}^{+}$ instead of $\mathrm{H}_{3} \mathrm{O}^{+}$ .

Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.

Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.

3.1.10 (HL)—Acid/base strength constants

Ka, Kb, pKa and pKb

Interpreting Ka and pKa

Assessment in practice

Proton Transfer Reactions Summary

Question 1