Question

Assertion: Phenolphthalein is an acid-base indicator.

Reason: In an acid-base titration, this indicator is used.

• A. Both Assertion and Reason are false.
• B. Assertion is false, but Reason is true.
• C. Both Assertion and Reason are true. The reason is the correct explanation of Assertion.
• D. Both Assertion and Reason are true. The reason is not the correct explanation of Assertion.

Answer 1

The correct option is D

Both Assertion and Reason are true. The reason is not the correct explanation of Assertion.

Explanation for the Correct option:

Assertion:-

• Phenolphthalein is a weak organic acid.
• It sometimes can also be represented as HPh.
  $\mathrm{HPh}\left(\mathrm{Colourless}\right)\rightleftharpoons {\mathrm{H}}^{+}\left(\mathrm{colourless}\right)+{\mathrm{Ph}}^{-}\left(\mathrm{Pink}\right)$
• Thus the assertion is true.

Explanation for the reason:-

• In an acidic medium, an excess of the ${\mathrm{H}}^{+}$ ions is present and that is why the equilibrium is towards the left which results in a colourless solution
• While in a basic medium, the ${\mathrm{OH}}^{-}$ ions present combine with ${\mathrm{H}}^{+}$ ions to form unionized water molecules.
• Thus the equilibrium is towards the right and hence the solution has a pink colour.
• Therefore, we can say that phenolphthalein is an acid-base indicator.
• Thus the reason is also true but it is not the correct explanation for the assertion.

Therefore, option (D) Both Assertion and Reason are true. The reason is not the correct explanation of Assertion is the correct answer.