: 200
The rule: indicator transition range must overlap the steep part of the titration curve at the equivalence point. Strong acid + Strong base: Equivalence at pH 7. Steep curve from pH ~4 to ~10. Either phenolphthalein (8.2-10) or methyl orange (3.1-4.4) works — both fall in the steep range. Weak acid + Strong base: Equivalence at pH > 7 (basic, ~8-9). Steep curve covers pH ~7 to ~10. Use phenolphthalein (8.2-10). Methyl orange would change too early (below pH 4.4 — long before equivalence). Strong acid + Weak base: Equivalence at pH < 7 (acidic, ~5). Steep curve covers pH ~4 to ~7. Use methyl orange (3.1-4.4). Phenolphthalein would change too late (above pH 8.2 — past equivalence, in the buffer region). Weak acid + Weak base: NO steep region at equivalence → no simple indicator works → use pH meter. Memory trick: "Phph for bases" (Phenolphthalein for basic equivalence, i.e., weak acid + strong base). "MO for acids" (Methyl Orange for acidic equivalence, i.e., strong acid + weak base). JEE frequently asks which indicator for a specific titration type.