Pattern 1: Le Chatelier Application (frequency: 1–2 Qs/year)

• Format: Gaseous equilibrium given; asked effect of pressure / temperature / catalyst / inert gas change.
• Strategy: Check $\Delta n$ first; check if stress is temperature (only one that changes K); check inert gas + volume/pressure condition.
• Trap: "Catalyst increases equilibrium constant" → WRONG (K unchanged, no shift).
• Trap: "Inert gas at constant V shifts equilibrium" → WRONG (no effect at const. V).

Pattern 2: pH Calculation (frequency: 1–2 Qs/year)

• Sub-type A: Weak acid — use [$H^{+}$] = √(Ka·C); verify α ≪ 1.
• Sub-type B: Buffer — use Henderson-Hasselbalch: pH = pKa + log([salt]/[acid]).
• Sub-type C: Ultra-dilute acid (10^{-7} or 10^{-8} M) — must include water ionisation contribution; pH ≈ 6.98 for 10^{-8} M HCl.
• Sub-type D: Ka × Kb = Kw — given Ka of acid, find Kb of conjugate base.

Pattern 3: Ksp and Solubility (frequency: 1 Q/year)

• Format: Ksp given; find s in pure water or in common ion solution.
• Strategy: ICE table; approximate s ≪ [common ion] when valid.
• Trap: Assuming solubility order follows Ksp order across different formula types.

Pattern 4: Kc/Kp Conversion (frequency: occasional)

• Format: Given Kc at temperature T, find Kp (or vice versa).
• Strategy: Find $\Delta n$ (gaseous only); apply Kp = Kc(RT)^$\Delta n$; use R = 0.0821.
• Note: Negative $\Delta n$ means Kp < Kc (e.g., Haber process).

Pattern 5: Salt Hydrolysis / Acid-Base Theory (frequency: 1 Q/year)

• Format: Identify pH nature of salt solution, or identify correct acid-base definition.
• Strategy: Apply "Weak side Wins" rule for salt hydrolysis; Lewis acid/base for non-aqueous or non-proton-transfer scenarios.

Time allocation strategy

• Numerical (Kp, pH, Ksp): 2–3 minutes; write ICE table and formula first.
• Conceptual (Le Chatelier, catalyst, theory): 30–45 seconds; use elimination.
• Most marks lost on: catalyst trap, inert gas trap, ultra-dilute acid trap.