The most suitable indicator for the reaction between NaOH and HCOOH is?

Analyze the question: NaOH is a strong base and HCOOH (methanoic acid) is a weak acid. We need the indicator that changes colour in the right pH range for this titration.

Concept:

• At the equivalence point, a strong base and a weak acid form a salt (HCOONa) that hydrolyses in water: HCOO⁻ + H₂O ⇌ HCOOH + OH⁻
• This makes the solution basic at the equivalence point (pH above 7, roughly 8 to 9).
• The indicator's colour-change range must include this pH.

Indicator ranges: | Indicator | pH range | Suitable for | |---|---|---| | Methyl orange | 3.1 – 4.4 | Strong acid + weak base | | Bromocresol (green) | about 3.8 – 5.4 | Acidic range | | Methyl red | 4.4 – 6.2 | Acidic range (strong acid + weak base) | | Phenolphthalein | 8.3 – 10.0 | Weak acid + strong base |

Solution:

1. The reaction is NaOH + HCOOH → HCOONa + H₂O.
2. The equivalence point lies in the basic region (pH about 8 to 9).
3. Only phenolphthalein changes colour in this range (pink in base, colourless in acid).
4. Methyl orange, methyl red and bromocresol change colour in acidic pH ranges, so they would change well before the equivalence point and give an inaccurate result.

Answer: D. phenolphthalein

Tip: Remember "Phenolphthalein for Poor (weak) acids with strong bases," and "Methyl orange/red for Mild (weak) bases with strong acids." For strong acid with strong base, almost any indicator works.