Question

What is the molecular formula of Marshall’s acid?
A.${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{5}}}$
B.${{\text{H}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{8}}$
C.${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{4}}$
D.${{\text{H}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{7}}$

Answer 1

Hint: There are different acids of sulphur in which it exists in different oxidation states. Sulphuric acid is known as “oil of vitriol”. Marshall’s acid contains two sulphur molecules in its 6th oxidation state.

Complete step by step answer:
${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{5}}}$ - This acid is called Peroxymonosulphuric acid or “Caro’s acid”. The other names of the acids are persulfuric acid and peroxydisulfuric acid. In this acid, the sulphur is its 6th oxidation state adopts the centre of the tetrahedral geometry and this acid is one of the strongest oxidising agents known and is highly explosive too.

${{\text{H}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{8}}$ - This acid is called peroxydisulfuric acid and also known as the “Marshall’s Acid” after the name of its founder professor Hugh Marshall. It contains sulphur in its 6th oxidation state and a peroxide group. This is also a very strong oxidising agent. Its salts are commonly called persulphates.

${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{4}}$ - The formula for the most commonly used laboratory agent, “Sulphuric acid”. In this acid too, the oxidation state of sulphur is (6) and this is also a strong oxidising agent. But in comparison to the above acids, this acid is not much explosive.

${{\text{H}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{7}}$ - This acid is formed when sulphur trioxide is mixed sulphuric acid during the production of sulphuric acid. The common name of this acid is “oleum” and it is also known as “fuming sulphuric acid” and “pyrosulphuric acid”.

Hence, the correct option is B.

Note:
Sulphuric acid has been used for the preparation of a large number of industrial chemicals. The oxidising nature of the sulphuric acid can be checked by dropping it over a piece of paper. The paper will slowly char and the cellulose would be oxidized to carbon.