Pattern 1: Phosphorus Oxoacid Basicity (Highest Frequency)

• Appears 3–4 times per 5-year cycle
• Question form: "The basicity of $H_{3}PO_{2}$ is ___" or "Which acid is monobasic: $H_{3}PO_{2}$, $H_{3}PO_{3}$, $H_{3}PO_{4}$?"
• Trap always present: one option states "tribasic because 3 H atoms"
• Master strategy: Memorise structures — $H_{3}PO_{2}$ (1 P-OH), $H_{3}PO_{3}$ (2 P-OH), $H_{3}PO_{4}$ (3 P-OH)

Pattern 2: Haber/Ostwald Process Conditions

• Appears in fill-in-the-blank or mixed statements format
• Tests: catalyst name (Fe vs Pt-Rh), temperature (450°C vs 500°C), pressure (200 atm for Haber)
• Common error: confusing Haber catalyst (Fe) with Ostwald Step 1 catalyst (Pt-Rh)
• Common error: saying Step 2 of Ostwald needs a catalyst (it does NOT)

Pattern 3: Nitrogen Oxide Properties

• Appears as matching or multi-statement true/false questions
• Tests: oxidation state, paramagnetic/diamagnetic, neutral/acidic, anhydride identification
• Key matches to memorise: $N_{2}O_{3}$ ↔ $HNO_{2}$; $N_{2}O_{5}$ ↔ $HNO_{3}$; NO and $N_{2}O$ are neutral; NO and $NO_{2}$ are paramagnetic

Pattern 4: Boric Acid Lewis vs Bronsted

• Classic conceptual question: "$H_{3}BO_{3}$ acts as acid because ___"
• Correct answer always involves Lewis acid mechanism (accepts $OH^{-}$)
• Wrong options always include Bronsted acid description (donates $H^{+}$)

Pattern 5: Diborane Bonding

• Tests nature of bridging bonds (3c-2e, not 3c-4e)
• May ask about number of bridging vs terminal H atoms (2 bridging, 4 terminal)

Pattern 6: $PCl_{5}$ vs $NCl_{5}$ Existence

• Tests: Why $NCl_{5}$ doesn't exist (no d orbitals in N valence shell)
• Related: $PCl_{5}$ geometry (trigonal bipyramidal, $sp^{3}$d) and axial vs equatorial bond lengths