Phenolphthalein is a :

strong acid

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weak acid

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(a) and (b) both

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none of the above

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Solution

The correct option is A weak acid
Phenolphthalein is an indicator which remains colourless in acidic and medium and changes it's colour to pink in basic medium. The structure of phenolphthalein has phenol ring so its weak acid able to lose H+.

It is mildly acidic in nature (i.e a weak acid) and hence it undergoes a colour change to pink in basic conditions.

Due to this property of phenolphthalein, it is also used in acid-base titration.
Hence, the correct answer is option (B).

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Q. (A) In general phenolphthalein is used as an indicator for the titration of weak acid

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Reason: At equivalent point, solution is basic.

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