Question

What is the molecular formula of oleum?
A. ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{3}}}$
B. ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_4}$
C. ${{\rm{H}}_{\rm{2}}}{{\rm{S}}_2}{{\rm{O}}_7}$
D. ${{\rm{H}}_{\rm{2}}}{{\rm{S}}_2}{{\rm{O}}_8}$

Answer 1

We know that, fuming sulfuric acid also known as oleum refers to solutions of different compositions of sulfur trioxide in ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}$ (sulfuric acid).

Complete step-by-step answer:
1.) In formation of oleum, first the oxidation of sulphur dioxide to sulphur trioxide takes place. Then, sulphur trioxide reacts with sulphuric acid to form oleum. Again dilution of oleum gives sulphuric acid.
Reaction of sulphuric acid with sulphur dioxide to form oleum.

${\rm{S}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to \underbrace {{{\rm{H}}_{\rm{2}}}{{\rm{S}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}}_{{\rm{Oleum}}}$

So, correct molecular formula of oleum is ${{\rm{H}}_{\rm{2}}}{{\rm{S}}_2}{{\rm{O}}_7}$.

2.) The formula for sulfurous acid is ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{3}}}$. So, ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{3}}}$ is not the correct formula of oleum. option A is not correct.
3.) We know that molecular formula of sulfuric acid is ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_4}$. Oleum can be obtained by reaction of sulphur trioxide with sulphuric acid. So, ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_4}$is the molecular formula of sulphur. Hence, option B is not correct.

Peroxydisulfuric acid is an inorganic compound whose molecular formula is ${{\rm{H}}_{\rm{2}}}{{\rm{S}}_2}{{\rm{O}}_8}$. So, ${{\rm{H}}_{\rm{2}}}{{\rm{S}}_2}{{\rm{O}}_8}$ is not the molecular formula of oleum. Hence, option D is incorrect.

Therefore, the correct option is (C).

Additional Information:
Oleum is obtained from sulphuric acid. Again oleum is used to produce sulphuric acid. Dilution of oleum produces sulphuric acid. Chemical reaction for dilution of oleum is,

$\underbrace {{{\rm{H}}_{\rm{2}}}{{\rm{S}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}}_{{\rm{Oleum}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{2}}\underbrace {{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}}_{{\rm{Sulfuric}}\;{\rm{acid}}}$

Note: Students might think that as oleum is also known as fuming sulphuric acid, its molecular formula is same as sulphuric acid,${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_4}$. Actually the correct molecular formula of oleum is ${{\rm{H}}_{\rm{2}}}{{\rm{S}}_{\rm{2}}}{{\rm{O}}_7}$.