Step 1: Understanding the Concept:
Pyrosulphuric acid, also known as disulphuric acid or oleum, has the chemical formula $H_2S_2O_7$.
The prefix "pyro-" in inorganic chemistry typically indicates an acid formed by the removal of one molecule of water from two molecules of the parent "ortho" acid.
For pyrosulphuric acid, the parent acid is sulphuric acid ($H_2SO_4$).
Step 2: Key Formula or Approach:
The synthesis logic is: $2 H_2SO_4 \rightarrow H_2S_2O_7 + H_2O$.
Structurally, this involves two sulfonic acid groups ($SO_3H$) linked together by a bridging oxygen atom. This creates a characteristic $S-O-S$ anhydride bridge.
Step 3: Detailed Explanation:
Let's evaluate each option to identify the correct structure:
- Option (A): $HOO-SO_2-OH$. This is peroxymonosulphuric acid ($H_2SO_5$), also known as Caro’s acid. It contains a peroxo ($-O-O-$) group instead of a bridging oxygen.
- Option (B): $HO-SO_2-O-O-SO_2-OH$. This is peroxodisulphuric acid ($H_2S_2O_8$), also known as Marshall’s acid. It contains a peroxide bridge ($S-O-O-S$) between two sulfur atoms.
- Option (C): $O^--SO_2-OH$. This is the bisulphate ion ($HSO_4^-$). It is an ionic species and not the structure of a neutral polyacid.
- Option (D): $HO-SO_2-O-SO_2-OH$. This structure shows two sulfur atoms, each bonded to two double-bonded oxygens and one hydroxyl group. The two sulfur atoms are connected by a single oxygen atom in the middle. This forms the $H_2S_2O_7$ molecule with an $S-O-S$ linkage. This is the correct structure for pyrosulphuric acid.
In this molecule, each sulfur atom is in the +6 oxidation state. The $S-O-S$ bond angle is approximately $120^\circ$.
Step 4: Final Answer:
Based on the chemical formula $H_2S_2O_7$ and the $S-O-S$ linkage requirement, the correct structure is option (D).