When .......... is passed through red hot iron tube at 878 K, benzene is formed.

Answer

To solve the question, we need to identify the compound that, when passed through a red-hot iron tube at 878 K, produces benzene (C₆H₆).

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- We know that benzene has the molecular formula C₆H₆.
- The question indicates that a certain hydrocarbon (let's call it X) is passed through a red-hot iron tube at a high temperature (878 K), resulting in the formation of benzene.

2. **Identifying the Hydrocarbon**:
- Benzene can be formed from the dehydrogenation of certain hydrocarbons.
- One common precursor for benzene is ethane (C₂H₆).

3. **Dehydrogenation Process**:
- When ethane is subjected to high temperatures in the presence of a catalyst (like red-hot iron), it can lose hydrogen atoms to form benzene.
- The reaction can be represented as:
\[ 3 \text{C₂H₆} \rightarrow \text{C₆H₆} + 3 \text{H₂} \]
- Here, three moles of ethane (C₂H₆) yield one mole of benzene (C₆H₆) and three moles of hydrogen gas (H₂).

4. **Conclusion**:
- Therefore, the compound that should be passed through the red-hot iron tube to produce benzene is ethane (C₂H₆).

### Final Answer:
When **ethane (C₂H₆)** is passed through a red-hot iron tube at 878 K, benzene (C₆H₆) is formed.

---