The titration curve of weak acid vs. strong base with...

Phenolphthalein is an organic acid and can be represented as $HPh$.

$\mathop{HPh}_{\text{(colourless)}} ⇌ {H}^{+} + \mathop{{Ph}^{-}}_{\text{(Pink)}}$

Using Henderson equation for phenolphthalein

$pH = {pK}_{In} + \log \left[\dfrac{\left[{Ph}^{-}\right]}{[ HPh ]}\right]$

At equivalence point \left[{Ph}^{-}\right] = [ HPh ]$</p><p>$∴ {pH}_{2} = {pK}_{In} = - log [ 4 ] + 10 = - 0 . 6 + 10 = 9 . 4$</p><p>Hence at$( 9 . 4 \pm 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.