JEE Chemistry 2023 Question with Solution

Given: The question asks for the incorrect statement about the use of indicators in acid-base titration.

Find: Which option gives an unsuitable indicator-choice pair.

Choose the indicator according to the pH range at the equivalence point.

From the solution:

• Methyl orange works in the acidic range $3.2 - 4.5$.
• Phenolphthalein works in the basic range $8.3 - 10.5$.

Therefore:

• Methyl orange is suitable for strong acid vs weak base titration.
• Phenolphthalein is suitable for weak acid vs strong base titration.
• Either indicator may be used for strong acid vs strong base titration depending on the sharp pH jump near the equivalence point.
• For weak acid vs weak base titration, methyl orange is not suitable.

Therefore, the incorrect statement is option A.

Given: Indicator ranges listed in the solution are:

Find: The statement that is incorrect.

In a strong acid vs weak base titration, the equivalence point lies in the acidic region, so methyl orange is suitable.

In a weak acid vs strong base titration, the equivalence point lies in the basic region, so phenolphthalein is suitable.

In a strong acid vs strong base titration, there is a sharp pH change near neutrality, so suitable common indicators may still be used.

But in a weak acid vs weak base titration, the pH change near equivalence is not sharp enough for methyl orange to be a proper choice.

Hence, the statement "Methyl orange may be used for a weak acid vs weak base titration" is incorrect.

So, the correct option is A.