Class 11 NEET Chemistry Equilibrium Questions Part 2

Concept:Phenolphthalein is an acid–base indicator that remains colourless in acidic or neutral medium and turns pink in basic medium.Chemical Equation / Formula:Titration: $\mathrm{2\,NaOH + H_2C_2O_4 \rightarrow Na_2C_2O_4 + 2\,H_2O}$Indicator: Phenolphthalein ( $\mathrm{HIn}$ colourless $\rightleftharpoons \mathrm{In^-}$ pink )Explanation:Sodium hydroxide is a strong base and oxalic acid is a weak acid.At the equivalence point, the solution becomes slightly basic due to hydrolysis of sodium oxalate.Phenolphthalein changes colour in the pH range $8.3 – 10.0$.Before the endpoint, the solution is acidic/neutral, so the indicator is colourless.As soon as the pH becomes slightly basic (close to equivalence point), the indicator turns pink.Therefore, the observable colour change is from colourless to pink.Shortcut:Remember: For strong base vs weak acid titrations, phenolphthalein shows colourless $\rightarrow$ pink (endpoint in basic medium).For strong acid vs strong base, it is also colourless $\rightarrow$ pink. Only in weak base vs strong acid titrations it remains colourless throughout.Answer:Option D: colourless to pink