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:

If the molar conductivity $\left(\Lambda_{\mathrm{m}}\right)$ of a $0.050 \mathrm{~mol} \mathrm{~L}^{-1}$ solution of a monobasic weak acid is $90 \mathrm{~S} \mathrm{~cm}^2 \mathrm{~mol}^{-1}$, its extent (degree) of dissociation will be

[Assume $\Lambda_{+}^{\circ}=349.6 \mathrm{~S} \mathrm{~cm}^2 \mathrm{~mol}^{-1}$ and $\Lambda_{-}^{\circ}=50.4 \mathrm{~S} \mathrm{~cm}^2 \mathrm{~mol}^{-1}$.]

Phosphoric acid ionizes in three steps with their ionization constant values $K_{a_1}, K_{a_2}$ and $K_{a_3}$, respectively, while $K$ is the overall ionization constant. Which of the following statements are true?

A. $\quad \log \mathrm{K}=\log \mathrm{K}_{\mathrm{a}_1}+\log \mathrm{K}_{\mathrm{a}_2}+\log \mathrm{K}_{\mathrm{a}_3}$

B. $\mathrm{H}_3 \mathrm{PO}_4$ is a stronger acid than $\mathrm{H}_2 \mathrm{PO}_4^{-}$and $\mathrm{HPO}_4^{2-}$

C. $K_{a_1}>K_{a_2}>K_{a_3}$

D. $K_{a_1}=\frac{K_{a_3}+K_{a_2}}{2}$

Choose the correct answer from the options given below :