MHT CET Chemistry Question (2026) — Solution

Step-by-step solution

The titration of NaCl against AgNO _3 using fluorescein indicator is an example of Fajan's method (precipitation titration). Fluorescein acts as an adsorption indicator. In the aqueous solution, it exists as the fluoresceinate anion, which imparts a pale yellow (or yellowish-green) colour to the solution. During the titration, AgCl precipitates. Before the equivalence point, the AgCl particles adsorb excess Cl ^- ions, acquiring a negative charge that repels the fluoresceinate anions. Just after the equivalence point, the addition of excess AgNO _3 causes the AgCl particles to adsorb Ag ^+ ions, acquiring a positive charge. The positively charged precipitate now strongly adsorbs the fluoresceinate anions. This adsorption deforms the electron cloud of the indicator, resulting in a distinct colour change to reddish pink on the surface of the precipitate. Thus, the colour change observed at the end point is from pale yellow to reddish pink. Answer: Pale yellow to reddish pink