The oxygen dissolved in water exerts a partial pressure of $20 \mathrm{kPa}$ in the vapour above water. The molar solubility of oxygen in water is ______ $\times {10}^{-5} \mathrm{mol} {\mathrm{dm}}^{-3}.$

(Round off to the Nearest Integer).

[Given : Henry's law constant $={\mathrm{K}}_{\mathrm{H}}=8.0\times {10}^4 \mathrm{kPa}$ for ${\mathrm{O}}_2.$ Density of water with dissolved oxygen$=1.0 \mathrm{kg} {\mathrm{dm}}^{-3}$]

${\mathrm{AB}}_2$ is $10\%$ dissociated in water to ${\mathrm{A}}^{2+}$ and ${\mathrm{B}}^{-}$. The boiling point of $10.0$ molal aqueous solution of ${\mathrm{AB}}_2$ is-- ${}^{\circ }\mathrm{C}$. (Round off to the Nearest Integer).

[Given : Molal elevation constant of water ${\mathrm{K}}_{\mathrm{b}}=0.5 \mathrm{K} \mathrm{kg}{ \mathrm{mol}}^{-1}$ boiling point of pure water$=100°\mathrm{C}$]

A $6.50$ molal solution of $\mathrm{KOH}\left(\mathrm{aq}.\right)$ has a density of $1.89 \mathrm{g}{ \mathrm{cm}}^{-3}$. The molarity of the solution is $\mathrm{mol} {\mathrm{dm}}^{-3}$. (Round off to the Nearest Integer).

[Atomic masses: $\mathrm{K}:39.0 \mathrm{u}; \mathrm{O}:16.0 \mathrm{u}; \mathrm{H}:1.0 \mathrm{u}$]