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Difficulty: EasyAcid-Base Titrations, Indicators, and Volumetric Calculations

Which of the following indicators is most suitable for detecting the end point in a volumetric titration of ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) against sodium hydroxide (NaOH\text{NaOH}) solution?

  1. PhenolphthaleinAnswer
  2. B
    Methyl orange
  3. C
    Methyl red
  4. D
    Screened methyl orange

Answer

Phenolphthalein is the most suitable indicator because the equivalence point of a weak acid (ethanoic acid) titrated against a strong base (sodium hydroxide) occurs in the basic pH range.
In the titration of ethanoic acid (a weak acid) with sodium hydroxide (a strong base), the salt formed at neutralization undergoes hydrolysis to yield an alkaline solution with a pH greater than 7. Phenolphthalein is the correct choice because its color change interval (pH 8.3–10.0) coincides with this basic equivalence point.

Step-by-Step Solution

1
Identify the nature of the acid and base involved in the titration.
Ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) is a weak acid, while sodium hydroxide (NaOH\text{NaOH}) is a strong base.
The relative strengths of the acid and base determine the pH of the salt solution at the equivalence point.
2
Determine the expected pH at the equivalence point.
The resulting salt (sodium ethanoate) undergoes hydrolysis to produce hydroxide ions, giving an alkaline equivalence point with a pH between 8 and 9.
Anions of weak acids hydrolyze in water to form basic solutions.
3
Match the equivalence point pH with the transition range of an indicator.
Phenolphthalein has a pH transition range of 8.3–10.0.
A suitable indicator must undergo a sharp color change in the pH region corresponding to the equivalence point of the reaction.

Key Concept

Indicator Selection for Acid-Base Titrations
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Topics

Acid-Base Titrations, Indicators, and Volumetric Calculations

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