In the manufacture of sulphuric acid by the contact process, sulphur trioxide is not directly dissolved in water. Why?

What happens when

$\left(\mathrm{A}\right)$ ${\mathrm{PCl}}_5$ is heated. 

$\left(\mathrm{B}\right)$ ${\mathrm{PCl}}_5$  is reacted with heavy water.

Complete and balance the following:

$\left(\mathrm{i}\right) {\mathrm{P}}_4{\mathrm{O}}_{10}+{\mathrm{PCl}}_5⟶$

$\left(\mathrm{ii}\right) {\mathrm{NH}}_3+\mathrm{NaOCl}\stackrel{\mathrm{Gelatin}}{\to }$