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Question

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


A

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

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B

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

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C

The assertion is true but the reason is false.

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D

Both assertion and reason are false.

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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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