(a) Give reasons for the following :
(i) `SF_(6)` is a well known compound but `SCl_(6)` is not known
(ii) In aqueous solution, HF is a weaker acid than HCl
(iii) Addition of `Cl_(2)` to KF solution gives it a brown colour but excess of `Cl_(2)` turns it colourless
(b) Draw the structures of
(i) Peroxodisulphuric acid
(ii) bromine trifluoride.