Question

Identify the correct molecular formula of 'Oleum' from following.

• A. \( \text{H}_2\text{S}_2\text{O}_3 \)
• B. \( \text{H}_2\text{S}_2\text{O}_7 \)
• C. \( \text{H}_2\text{S}_2\text{O}_5 \)
• D. \( \text{H}_2\text{S}_2\text{O}_8 \)

Hint:

Oleum = \(H_2SO_4 + SO_3\); just add the atoms together.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Oleum is an important industrial chemical, particularly in the manufacture of sulfuric acid via the Contact process.
It is also known as fuming sulfuric acid.
The question requires recalling the specific chemical composition and molecular formula of this substance.
Step 2: Key Formula or Approach:
Relate the common name "Oleum" to its chemical composition.
Oleum is formed by dissolving sulfur trioxide gas into concentrated sulfuric acid.
Write the chemical equation for this combination to find the resulting formula.
Step 3: Detailed Explanation:
Oleum is essentially a solution of sulfur trioxide (\( \text{SO}_3 \)) in anhydrous sulfuric acid (\( \text{H}_2\text{SO}_4 \)).
When these two combine, they form a new oxoacid of sulfur:
\[ \text{H}_2\text{SO}_4 + \text{SO}_3 \rightarrow \text{H}_2\text{S}_2\text{O}_7 \]
The chemical name for \( \text{H}_2\text{S}_2\text{O}_7 \) is pyrosulfuric acid or disulfuric acid. "Oleum" is its common, historical, and commercial name.
Let's identify the other formulas for clarity:
(A) \( \text{H}_2\text{S}_2\text{O}_3 \): Thiosulfuric acid.
(C) \( \text{H}_2\text{S}_2\text{O}_5 \): Disulfurous acid.
(D) \( \text{H}_2\text{S}_2\text{O}_8 \): Peroxydisulfuric acid (Marshall's acid).
Therefore, the correct formula for oleum is \( \text{H}_2\text{S}_2\text{O}_7 \).
Step 4: Final Answer:
The molecular formula of Oleum is \( \text{H}_2\text{S}_2\text{O}_7 \).