(a) (i) `P_(4)+10" " SO_(2)Cl_(2) rightarrow 4 PCl _(5) + 10" "SO_(2)`
(ii) `6" "XeF_(4)+12H_(2)Orightarrow2 " "eO_(3) + 4" "XeO_(3)+24HF +3O_(2)`
b (i)Because down the group, +3 oxidation state becomes more and more stable due to higher energy involved to unpair the electrons or due to inert pair effect.
(ii) Due to the formation of `[PCl_(4)]^(+)[PCI_(6)]^(-)`
(iii) Because halogens readily accept an electron