In the preparation of ${\mathrm{H}}_2{\mathrm{SO}}_4$

In the contact process, sulphur trioxide is prepared from sulphur dioxide.

Sulphur trioxide is mixed with sulphuric acid giving us oleum which on hydrolysis gives us concentrated sulphuric acid the reaction goes as follows.

${\mathrm{SO}}_{3( \mathrm{g})}+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{H}}_2{ \mathrm{S}}_2{\mathrm{O}}_7$

on diluting

${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}_2+2{\mathrm{SO}}_4$