(i) Dilution of a strong concentrated acid is an exothermic reaction. Hence, we must add acid to water dropwise to control the heat evolved in the reaction. If you add water to acid, enormous amount of heat will be released causing the acid to splash out and cause burns.
(ii)The salt formed is sodium sulphate. Its $\mathrm{pH}$ is 7 as it is a neutral salt formed by the neutralisation of a strong acid and a strong base.
(iii) Only the ions are responsible for the $\mathrm{pH}$ of a substance. Hydrogen ions in $\mathrm{HCl}$ are produced in the presence of water. The separation of ${\mathrm{H}}^{+}$ion from $\mathrm{HCl}$ molecules cannot occur in the absence of water. Hence, dry $\mathrm{HCl}$ does not give characteristics of an acid as ${\mathrm{H}}^{+}$ions are not present in it.