JEE Main · 2023 · 08 Apr 2023 · Shift 1 · Equilibrium

An indicator changes colour over a range centred on its $\mathrm{p}{K}_{\mathrm{In}}$ and spanning about $\mathrm{p}{K}_{\mathrm{In}}\pm 1$. Here $K_{\mathrm{In}}=4\times 10^{-10}$, so $\mathrm{p}{K}_{\mathrm{In}}=-\log (4\times 10^{-10})=10-\log 4=10-0.6=9.4$, giving a range of roughly $8.4$ to $10.4$. Statement (A) is wrong, since a weak acid–weak base titration has no sharp pH jump for phenolphthalein to detect. Statement (B) is right: the colour begins to appear near pH $8.4$, the lower end of this range. Statement (C) is wrong, because phenolphthalein is a weak organic acid, not a base. Statement (D) is right: in acidic medium it stays in its colourless lactone form. Two of the statements are correct.