Solubility product of $\mathrm{CaC}_2 \mathrm{O}_4$ at a given temperature in pure water is $4 \times 10^{-9}\left(\mathrm{~mol} \mathrm{~L}^{-1}\right)^2$. Solubility of $\mathrm{CaC}_2 \mathrm{O}_4$ at the same temperature is

A weak acid with $$\mathrm{p} K_a ~5.9$$ and weak base with $$\mathrm{p} K_b ~5.8$$ are mixed in equal proportions. $$\mathrm{pH}$$ of the resulting solution is

Which among the following has highest $$\mathrm{pH}$$ ?

$$K_a$$ values for acids $$\mathrm{H}_2 \mathrm{SO}_3, \mathrm{HNO}_2, \mathrm{CH}_3 \mathrm{COOH}$$ and $$\mathrm{HCN}$$ are respectively $$13 \times 10^{-2}, 4 \times 10^{-4}, 1.8 \times 10^{-5}$$ and $$4 \times 10^{-10}$$, which of the above acids produces stronger conjugate base in aqueous solution?