$$ \text { The correct order of solubility of the given salts in water at } 298 \mathrm{~K} \text { is } $$

$$ \begin{array}{|l|l|} \hline \text { Salt } & \mathbf{K}_{\text {sp }} \text { at } \mathbf{2 9 8} \text { K } \\ \hline \mathrm{AgBr} & 5.0 \times 10^{-13} \\ \hline \mathrm{Zn}(\mathrm{OH})_2 & 1.0 \times 10^{-15} \\ \hline \mathrm{Hg}_2 \mathrm{Cl}_2 & 1.3 \times 10^{-18} \\ \hline \end{array} $$

In a qualitative analysis, $\mathrm{Bi}^{3+}$ is detected by appearance of precipitate of $\mathrm{BiO}(\mathrm{OH})(\mathrm{s})$. Calculate pH when the following equilibrium exists at 298 K .

$$ \mathrm{BiO}(\mathrm{OH})(\mathrm{s}) \rightleftharpoons \mathrm{BiO}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}), \mathrm{K}=4 \times 10^{-10} $$

(Given : $\log 2=0.3010$ )

At 298 K , a certain buffer solution contains equal concentrations of $\mathrm{X}^{-}$and $\mathrm{HX}, \mathrm{K}_{\mathrm{b}}$ for $\mathrm{X}^{-}$is $10^{-10}$. What is the pH of this buffer solution?

Phenolphthalein is used as an indicator for the titration of sodium hydroxide solution against a standard solution of oxalic acid. The colour change that is observed at an alkaline pH close to the equivalence point during this titration is: