A compound 'X' is a weak acid and it exhibits colour change at pH close to the equivalence point during neutralization of NaOH with CH₃ COOH. Compound 'X' exists in ionized form in basic medium. The compound 'X' is

Solution

Chemistry ma’am · 3 steps

Find the pH at the equivalence point first, then pick the indicator whose range sits on it.

1. Equivalence point pH. At the equivalence point the flask holds only CH₃COONa. The acetate ion hydrolyses, CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻, so the solution is basic, with pH about 8 to 9.
2. How the indicator works. An acid-base indicator is a weak acid, HIn ⇌ H⁺ + In⁻. Base removes H⁺, so the ionized form In⁻ dominates in basic medium. The colour changes over roughly pK_(In) ± 1, and that range must contain the equivalence point pH.

   Indicator	Working pH range	Suits
   Methyl orange	3.1 to 4.4	strong acid and weak base
   Methyl red	4.4 to 6.2	strong acid and weak base
   Phenolphthalein	8.2 to 10	weak acid and strong base
   Eriochrome Black T	used near pH 10 with EDTA	metal-ion (complexometric) titrations

3. So. Phenolphthalein is a weak acid that turns from colourless to pink in its ionized form, and its range covers the basic equivalence point of CH₃COOH with NaOH. The answer is (c).

The idea to keep. Weak acid with strong base gives a basic equivalence point, so use phenolphthalein. Strong acid with weak base gives an acidic one, so use methyl orange or methyl red.

Answer from the NTA final answer key. Solution by PrepWiser.