`Mg^(2+)` ions is essential for selective precipitation of `Fe(OH)_(3)` by aqueous `NH_(3)`.

Explanation

To determine whether the statement "Mg^(2+) ions is essential for selective precipitation of Fe(OH)_(3) by aqueous NH_(3)" is true or false, we can analyze the situation step by step.

### Step-by-Step Solution:

1. **Understanding Aqueous NH3**:
- Aqueous NH3 can be represented as NH4OH (ammonium hydroxide) when dissolved in water. This means that when NH3 is in solution, it acts as a weak base and can produce OH⁻ ions.

2. **Dissociation of NH4OH**:
- The dissociation of NH4OH in water can be written as:
\[ NH4OH \rightleftharpoons NH4^+ + OH^- \]
- This reaction indicates that NH4OH produces hydroxide ions (OH⁻), which are necessary for the precipitation of metal hydroxides.

3. **Formation of Fe(OH)3**:
- Iron(III) ions (Fe³⁺) can react with hydroxide ions to form iron(III) hydroxide:
\[ Fe^{3+} + 3OH^- \rightarrow Fe(OH)_{3(s)} \]
- This indicates that Fe(OH)3 will precipitate out of the solution when sufficient OH⁻ ions are present.

4. **Role of Mg²⁺ Ions**:
- Magnesium ions (Mg²⁺) can also react with hydroxide ions to form magnesium hydroxide:
\[ Mg^{2+} + 2OH^- \rightarrow Mg(OH)_{2(s)} \]
- The solubility product (Ksp) of Fe(OH)3 is lower than that of Mg(OH)2, meaning Fe(OH)3 will precipitate out of solution before Mg(OH)2 when OH⁻ concentration is increased.

5. **Selective Precipitation**:
- The presence of Mg²⁺ ions can affect the precipitation process. However, the key point is that the selective precipitation of Fe(OH)3 can occur in the presence of NH4OH without the need for Mg²⁺ ions.
- The lower Ksp of Fe(OH)3 allows it to precipitate first, even if Mg²⁺ ions are present.

6. **Conclusion**:
- Therefore, the statement that "Mg²⁺ ions is essential for selective precipitation of Fe(OH)3 by aqueous NH3" is **false**. The precipitation of Fe(OH)3 can occur independently of Mg²⁺ ions due to the differences in their solubility products.

### Final Answer:
The statement is **false**.