When fused NaCl undergoes electrolysis the product formed at cathode is

Which element from following exhibits highest number of various different possible oxidation states?

Which among the following has lowest boiling point?

A buffer solution is prepared by mixing 0.01 M HCN and 0.02 MNaCN . If $\mathrm{K}_{\mathrm{a}}$ for HCN is $6.6 \times 10^{-10}$, what is the concentration of $\mathrm{H}^{+}$ions in solution?