In `NH_3` solution of `Zn^(2+),Zn^(2+) " form" [Zn(NH_3)_4]^(+2)` In this solution , to increase the concentration of `Zn^(2+)` we have to add -

Options

A. `H_2O`
B. `HCl`
C. `NH_3`
D. Either `H_2O " or " HCl`

Answer

To solve the problem, we need to determine which substance to add to an `NH3` solution of `Zn^(2+)` to increase the concentration of `Zn^(2+)`. The complex ion formed in this solution is `[Zn(NH3)4]^(2+)`.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The equilibrium in the solution can be represented as:
\[ Zn^{2+} + 4 NH_3 \rightleftharpoons [Zn(NH_3)_4]^{2+} \]
Here, `Zn^(2+)` ions react with ammonia to form the complex ion.

2. **Le Chatelier's Principle**:
According to Le Chatelier's principle, if a change is made to a system at equilibrium, the system will adjust to counteract that change and restore a new equilibrium.

3. **Adding Water**:
If we add water to the solution, it will dilute the concentration of all species. However, since the left side (reactants) has more species (1 `Zn^(2+)` and 4 `NH3` = 5 species) compared to the right side (1 `[Zn(NH3)4]^(2+)`), the equilibrium will shift to the left to increase the concentration of `Zn^(2+)`. Thus, adding water will increase the concentration of `Zn^(2+)`.

4. **Adding HCl**:
When HCl is added, it dissociates into `H^+` and `Cl^-`. The `Cl^-` ions can react with `Zn^(2+)` to form `ZnCl2`, which decreases the concentration of `Zn^(2+)`. However, according to Le Chatelier's principle, the system will shift to the left to counteract the decrease in `Zn^(2+)`, thus increasing its concentration.

5. **Adding Ammonia**:
If we add more ammonia (`NH3`), it will increase the concentration of the reactants. According to Le Chatelier's principle, the equilibrium will shift to the right to form more `[Zn(NH3)4]^(2+)`, which will decrease the concentration of `Zn^(2+)`.

6. **Conclusion**:
The substances that can be added to increase the concentration of `Zn^(2+)` are water and HCl. Adding ammonia will not help in increasing `Zn^(2+)` concentration.

### Final Answer:
To increase the concentration of `Zn^(2+)`, we can add either **water** or **HCl**.