The incorrect statement for the use of indicators in acid-base titration is :

Solution

Chemistry ma’am · 3 steps

An indicator works only if its colour-change range sits inside the steep pH jump near equivalence.

1. How indicators work. Near the equivalence point, one drop of titrant makes the pH jump sharply. An indicator is suitable only if its colour changes somewhere inside that jump. Methyl orange changes over pH 3.1 to 4.4, and phenolphthalein over pH 8.3 to 10.

   Titration	Steep pH jump	Methyl orange (3.1 to 4.4)	Phenolphthalein (8.3 to 10)
   Strong acid vs strong base	about 3 to 11	works	works, so (d) is true
   Weak acid vs strong base	about 7 to 11	fails	works, so (a) is true
   Strong acid vs weak base	about 3 to 7	works, so (c) is true	fails
   Weak acid vs weak base	no sharp jump	fails, so (b) is false	fails

2. Weak with weak. When both the acid and the base are weak, the pH changes gradually through the equivalence point. No indicator changes colour sharply at one drop, so methyl orange cannot give a clear end point.
3. So. Statements (a), (c) and (d) are correct. The incorrect statement is (b).

The idea to keep. Match the indicator to the pH jump: strong base side, use phenolphthalein; strong acid side, use methyl orange; strong with strong, either works; weak with weak, no indicator is reliable.

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