Q71 / 180 · AIIMS 2018 (26 May) · Chemistry

When $\mathrm{CH}_3 \mathrm{COOCH}_3+\mathrm{HCl}$ is titrated with $\mathrm{NaOH}$ then at neutral point the colour of phenolphthalein becomes colourless from pink due to

  • A
    formation of $\mathrm{CH}_3 \mathrm{OH}$
  • Correct answer
    B
    formation of $\mathrm{CH}_3 \mathrm{COOH}$ which acts as a weak acid
  • C
    phenolphthalein vaporizes
  • D
    presence of $\mathrm{HCl}$.

Solution

$\mathrm{NaOH}$ solution is taken in a conical flask with phenolphthalein indicator (pink colour) and $\mathrm{CH}_3 \mathrm{COOCH}_3+\mathrm{HCl}$ solution is added from burette slowly. First, $\mathrm{HCl}$ left after hydrolysis of ester (being a strong acid) gets consumed then $\mathrm{CH}_3 \mathrm{COOH}$ being a weak acid will bring the titration to the neutralisation point. $\begin{aligned} & \mathrm{CH}_3 \mathrm{COOCH}_3 \frac{\mathrm{H}_2 \mathrm{O}}{\mathrm{H}^{+}} \rightarrow \mathrm{CH}_3 \mathrm{COOH}+\mathrm{CH}_3 \mathrm{OH} \\ & \mathrm{HCl}+\mathrm{NaOH} \longrightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O} \\ & \mathrm{CH}_3 \mathrm{COOH}+\mathrm{NaOH} \longrightarrow \mathrm{CH}_3 \mathrm{COONa}\end{aligned}$

Hence, the correct answer is option: 'B'.

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