The reagents, `NH_(4)Cl` and aqueous `NH_(3)` will precipitate

Options

A. `Ca^(2+)`
B. `Al^(3+)`
C. `Mg^(2+)`
D. `Zn^(2+)`

Answer

To determine which ion will precipitate when mixing `NH4Cl` and aqueous `NH3`, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reagents**:
The reagents provided are `NH4Cl` (ammonium chloride) and `NH3` (ammonia in aqueous solution).

2. **Understand the Reaction**:
When `NH3` is dissolved in water, it forms ammonium hydroxide, which can be represented as `NH4OH`. The reaction can be summarized as:
\[ NH3 + H2O \rightleftharpoons NH4^+ + OH^- \]

3. **Consider the Ions in Solution**:
The ions present in the solution will be:
- From `NH4Cl`: `NH4^+` and `Cl^-`
- From `NH3`: `NH4^+` and `OH^-`

4. **Formation of Hydroxides**:
The presence of `OH^-` ions can lead to the precipitation of metal hydroxides. In this case, we are interested in which metal ions can react with `OH^-` to form a precipitate.

5. **Identify Possible Precipitates**:
Common metal ions that form insoluble hydroxides include:
- `Al^3+` (aluminum ion) which forms `Al(OH)3`
- `Fe^3+` (iron(III) ion) which forms `Fe(OH)3`
- `Cu^2+` (copper(II) ion) which forms `Cu(OH)2`
- `Zn^2+` (zinc ion) which forms `Zn(OH)2`

6. **Determine Which Ion Precipitates**:
Among the ions listed, `Al^3+` will precipitate as `Al(OH)3` when mixed with the hydroxide ions from the `NH3` solution. The reaction can be represented as:
\[ Al^{3+} + 3OH^- \rightarrow Al(OH)3 \downarrow \]

7. **Conclusion**:
Therefore, the ion that will precipitate when `NH4Cl` and aqueous `NH3` are mixed is `Al^3+`, forming `Al(OH)3`.

### Final Answer:
The ion that will precipitate is `Al^3+` as `Al(OH)3`.

---