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Question

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


  1. Assertion and reason are true and the reason is the correct explanation of assertion

  2. Both assertion and reason are true but the reason is not the correct explanation of assertion

  3. The assertion is true but the reason is false.

  4. Both assertion and reason are false.


Solution

The correct option is B

Both assertion and reason are true but the reason is not the correct explanation of assertion


Phenolphthalein is a weak organic acid and may be represented as HPh.
HPh(Colourless)    H+(colourless)  +  Ph(Pink)

In acidic medium, an excess of H+ ions are present and so equilibrium is towards left and hence the solution is colourless. While in the basic medium, OH ions combine with H+ ions to form unionised water molecules and so equilibrium is towards the right and hence solution has a pink colour. Therefore, phenolphthalein is an acid-base indicator.

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