Phenolphthalein is an organic acid and can be represented as HPh.
(colourless) HPh⇌H++ (Pink) Ph−
Using Henderson equation for phenolphthalein
pH=pKIn+log[[HPh][Ph−]]
At equivalence point [Ph−]=[HPh]
∴pH2=pKIn=−log[4]+10=−0.6+10=9.4
Hence at (9.4±1)PH , phenolphthalein starts changing colour. Phenolphthalein is colourless in acidic medium and pink in basic medium. Phenolphthalein indicator distinguish the pH change between 8 to 10. Therefore, it is used for strong acid and strong base titration or weak acid and strong base titration. Hence, A and C are incorrect statements.
