A compound 'X' is a weak acid and it exhibits colour change at pH close to the equivalence point during neutralization…
Redox Reactions · Class 11 · JEE Main Previous Year Question
A compound 'X' is a weak acid and it exhibits colour change at pH close to the equivalence point during neutralization of NaOH with . Compound 'X' exists in ionized form in basic medium. The compound 'X' is
- a
methyl orange
- b
methyl red
- c✓
phenolphthalein
- d
erichrome Black T
phenolphthalein
Step 1 — Identify the equivalence point pH:
For titration of (weak acid) with NaOH (strong base):
- At equivalence point, only is present
- pH at equivalence point ≈ 8.7–9 (basic)
Step 2 — Identify the indicator:
| Indicator | Type | pH range | Colour change | |---|---|---|---| | Methyl orange | Weak base | 3.1–4.4 | Red → Yellow | | Methyl red | Weak acid | 4.4–6.2 | Red → Yellow | | Phenolphthalein | Weak acid | 8.3–10 | Colourless → Pink | | Erichrome Black T | Used in EDTA titrations | — | — |
Step 3 — Match criteria:
- Weak acid ✓ (phenolphthalein is a weak acid)
- Colour change near pH 8.7–9 ✓ (phenolphthalein changes at pH 8.3–10)
- Exists in ionised form in basic medium ✓ (forms in base, giving pink colour)
Answer: Option (3) — Phenolphthalein
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