A reddish brown sol `("containing" Fe^(3+))` is obtained by:

Options

A. the addition of small amount of `FeCl_(3)` solution to freshly prepared `Fe(OH)_(3)` precipitate
B. the addition of `Fe(OH)_(3)` to freshly prepared `FeCl_(3)` solution
C. the addition of `NH_(4)OH` to `FeCl_(3)` solution dropwise
D. the addition of `NaOH to `FeCl_(3)` solution dropwise

Explanation

To solve the question regarding the reddish brown sol containing Fe³⁺ ions, we will analyze the process step by step.

### Step-by-Step Solution:

1. **Understanding the Components**:
- We are looking for a reddish brown sol that contains Fe³⁺ ions. The reddish brown color is characteristic of iron(III) compounds.

2. **Identifying the Precipitate**:
- Ferric hydroxide, which is represented as Fe(OH)₃, is a common precipitate formed when iron(III) ions react with hydroxide ions. This precipitate is typically reddish brown in color.

3. **Preparation of Ferric Hydroxide**:
- To prepare Fe(OH)₃, we can add a base (like NaOH) to a solution of FeCl₃ (ferric chloride). The reaction can be represented as:
\[ \text{FeCl}_3 + 3\text{NaOH} \rightarrow \text{Fe(OH)}_3 \downarrow + 3\text{NaCl} \]
- Here, Fe(OH)₃ precipitates out of the solution.

4. **Adding Ferric Chloride**:
- The question states that we need to obtain a reddish brown sol containing Fe³⁺. This can be achieved by adding a small amount of FeCl₃ solution to the freshly prepared Fe(OH)₃ precipitate.
- The addition of FeCl₃ will dissolve some of the Fe(OH)₃ and release Fe³⁺ ions into the solution, resulting in a colloidal sol.

5. **Conclusion**:
- Therefore, the reddish brown sol containing Fe³⁺ ions is obtained by the addition of a small amount of FeCl₃ solution to freshly prepared ferric hydroxide precipitate.

### Final Answer:
The reddish brown sol containing Fe³⁺ is obtained by adding a small amount of FeCl₃ solution to freshly prepared ferric hydroxide precipitate.

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