The best indicator to use for the titration of ethanoic acid with sodium hydroxide is

Step 1: Analyze the question Ethanoic acid (CH₃COOH) is a weak acid, and sodium hydroxide (NaOH) is a strong base. We need the indicator that changes colour at the right pH for this pairing.

Step 2: The concept The indicator must change colour within the steep part of the titration curve, around the equivalence point.

At the equivalence point, the product is sodium ethanoate (CH₃COONa), a salt of a weak acid and a strong base. It hydrolyses in water:

CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻

This produces excess OH⁻ ions, so the solution is slightly basic, with a pH of about 8.5 to 9.

Step 3: Match with the indicators

| Indicator | pH range | Suitable? | |---|---|---| | Methyl orange | 3.1 – 4.4 | No (acidic range) | | Screened methyl orange | about 3.2 – 4.2 | No (acidic range) | | Methyl red | 4.4 – 6.2 | No (too acidic) | | Phenolphthalein | 8.3 – 10.0 | Yes, matches the equivalence point |

Step 4: Answer Phenolphthalein changes colour (colourless in acid to pink in base) in the pH range of 8.3 to 10, which covers the equivalence point of a weak acid and strong base titration. The correct answer is B.

Exam tip: Remember the pairings:

• Strong acid + strong base: almost any indicator works
• Weak acid + strong base: phenolphthalein
• Strong acid + weak base: methyl orange
• Weak acid + weak base: no suitable indicator

A quick memory trick: "Phenolphthalein is for Poor (weak) acids."