Pattern 1: Le Chatelier's Principle Application (1–2 Qs/year)

Common format: "For the reaction X(g) ⇌ Y(g) at equilibrium, what happens when [pressure / temperature / catalyst / inert gas] is changed?"

Most tested traps:

• Catalyst → answer is always "no effect on equilibrium position or K"
• Inert gas at constant V → no effect (partial pressures unchanged)
• Inert gas at constant P → shifts toward more gas moles

Trick recognition: If the question says "inert gas is added," immediately look for whether volume is constant or pressure is constant.

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

Sub-patterns:

• Weak acid pH: given Ka and concentration → [$H^{+}$] = √(Ka·C)
• Buffer pH: given Ka, [acid], [salt] → Henderson-Hasselbalch
• Ultra-dilute acid: 10^{-7} or 10^{-8} M HCl → cannot ignore water ionization
• Ka × Kb = Kw trap: given Ka of acid, find Kb of conjugate base

Frequently tested values:

• Ka of acetic acid = $1.8 \times 10^{-5}$; pKa = 4.74
• Kb of $NH_{3}$ = $1.8 \times 10^{-5}$; pKb = 4.74
• Kw = 10^{-14} at 25°C

Pattern 3: Ksp and Common Ion (1 Q/year)

Format: "Ksp of [salt] is X. Find solubility in pure water / in [common ion] solution."

Key technique:

• Pure water: set up ICE table, express Ksp in terms of s
• Common ion: treat common ion concentration as essentially constant (s ≪ [common ion])

Pattern 4: Kc/Kp Conversion (occasional)

$K_p = K_c(RT)^{\Delta n}$

Steps: (1) Count only gaseous moles for $\Delta n$; (2) use R = 0.0821 L·atm/mol·K; (3) substitute T in Kelvin

Pattern 5: Salt Hydrolysis (occasional)

Format: "Identify the pH of a solution of [salt]"

Rule: Check parent acid and base strengths → apply the "Strong-Strong Neutral, Weak side Wins" rule.