A compound of iron exists as a dimer in vapour state. It is hygroscopic in nature and dissolves in water giving brown acidic solution. The compound is

Options

A. `Fe_(3)O_(4)`
B. `FeSO_(4)`
C. `FeCl_(3)`
D. `FeCl_(2)`

Explanation

To solve the question regarding the compound of iron that exists as a dimer in the vapor state, is hygroscopic, and dissolves in water to give a brown acidic solution, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Characteristics of the Compound**:
- The compound is a dimer in the vapor state.
- It is hygroscopic (absorbs moisture from the air).
- It dissolves in water to produce a brown acidic solution.

2. **Consider Possible Compounds of Iron**:
- The most common iron compounds that fit these characteristics are iron(III) chloride (FeCl3) and iron(II) chloride (FeCl2).
- However, iron(II) chloride does not typically form a brown solution when dissolved in water.

3. **Examine Iron(III) Chloride (FeCl3)**:
- FeCl3 is known to exist as a dimer in the vapor state as Fe2Cl6.
- It is hygroscopic, meaning it can absorb moisture from the environment.
- When dissolved in water, FeCl3 produces a brown solution due to the formation of ferric hydroxide (Fe(OH)3), which is brown in color.

4. **Reaction of FeCl3 with Water**:
- The dissolution of FeCl3 in water can be represented as:
\[ \text{FeCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3 \downarrow + 3\text{HCl} \]
- Here, Fe(OH)3 precipitates as a brown solid, and hydrochloric acid (HCl) is formed, which makes the solution acidic.

5. **Conclusion**:
- Based on the characteristics and the reactions observed, the compound that fits all the criteria is **iron(III) chloride (FeCl3)**.

### Final Answer:
The compound is **iron(III) chloride (FeCl3)**.

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