word_count: 250

Strong acid: pH = -log(C). Strong base: pH = 14 + log(C). Weak acid: pH = (1/2)(pKa + pC) where pC = -log(C). Weak base: pOH = (1/2)(pKb + pC), then pH = 14 - pOH. Acidic buffer: pH = pKa + log([salt]/[acid]) — Henderson-Hasselbalch. Basic buffer: pOH = pKb + log([salt]/[base]). Salt of weak acid + strong base: pH = 7 + (1/2)pKa + (1/2)log(C). Salt of strong acid + weak base: pH = 7 - (1/2)pKb - (1/2)log(C). Salt of weak acid + weak base: pH = 7 + (1/2)pKa - (1/2)pKb (independent of concentration). Amphoteric species: pH = (1/2)(pKa1 + pKa2). Very dilute acids (C < 10^-6 M): include water autoionisation, [H+] = C + sqrt(Kw), approximately. For diprotic acid H2A: use Ka1 for pH; [A^2-] approximately equals Ka2. Conjugate pair relationship: Ka * Kb = Kw = 10^-14. Key conversions: pKa + pKb = pKw = 14; pH + pOH = 14.