Oleum is formed when .............is passed through conc. `H_(2)SO_(4)`.

Explanation

**Step-by-Step Solution:**

1. **Understanding Oleum**: Oleum is a solution of sulfur trioxide (SO₃) in sulfuric acid (H₂SO₄). Its chemical formula is often represented as H₂S₂O₇, which can also be referred to as Pyro-sulfuric acid.

2. **Identifying the Reaction**: Oleum is formed when sulfur trioxide (SO₃) is dissolved in concentrated sulfuric acid (H₂SO₄). The reaction can be represented as:
\[ SO_3 + H_2SO_4 \rightarrow H_2S_2O_7 \]
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