Chemistry - Ionic Equilibrium Question with Solution | TestHub

Phenolphthalein is an organic acid and can be represented as $\mathrm{HPh}$.

$\underset{\text{ (colourless) }}{\mathrm{HPh}}\rightleftharpoons {\mathrm{H}}^{+}+\underset{\text{ (Pink) }}{{\mathrm{Ph}}^{-}}$

Using Henderson equation for phenolphthalein

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

At equivalence point  $\left[{\mathrm{Ph}}^{-}\right]=\left[\mathrm{HPh}\right]$

$\therefore {\mathrm{pH}}_2={\mathrm{pK}}_{\mathrm{In}}=-\log \left[4\right]+10=-0.6+10=9.4$

Hence at $(9.4\pm 1)\mathrm{PH}$ , 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.