Assess students' ability to analyze pH behavior of multi‑component acid–base solutions using equilibrium calculations and the Henderson–Hasselbalch equation. Tasks should include calculating pH of buffer mixtures and mixtures of weak acids/bases and their conjugates, predicting pH changes after addition of strong acid/base or dilution, setting up and solving K_a/K_b equilibrium expressions for competing species, and evaluating buffer capacity and the validity of common approximations; require numeric problem-solving and justification of assumptions.