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 of increasing the concentration of \( \text{Zn}^{2+} \) in a solution where \( \text{Zn}^{2+} \) forms the complex ion \( [\text{Zn(NH}_3)_4]^{2+} \), we can follow these steps:

### Step 1: Understand the Reaction
In the solution, the zinc ions \( \text{Zn}^{2+} \) react with ammonia \( \text{NH}_3 \) to form the complex ion \( [\text{Zn(NH}_3)_4]^{2+} \). The reaction can be represented as:
\[ \text{Zn}^{2+} + 4 \text{NH}_3 \rightleftharpoons [\text{Zn(NH}_3)_4]^{2+} \]

### Step 2: Apply Le Chatelier's Principle
According to Le Chatelier's principle, if we want to increase the concentration of \( \text{Zn}^{2+} \), we need to shift the equilibrium to the left. This can be achieved by either diluting the solution or removing some of the product.

### Step 3: Consider the Effects of Adding Water
Adding water (dilution) will decrease the concentration of the complex ion \( [\text{Zn(NH}_3)_4]^{2+} \) and shift the equilibrium to the left, thereby increasing the concentration of \( \text{Zn}^{2+} \):
\[ \text{Adding H}_2\text{O} \rightarrow \text{Shift to left} \rightarrow \text{Increase } [\text{Zn}^{2+}] \]

### Step 4: Consider the Effects of Adding HCl
Adding hydrochloric acid \( \text{HCl} \) will dissociate into \( \text{H}^+ \) and \( \text{Cl}^- \). The \( \text{Zn}^{2+} \) ions can react with \( \text{Cl}^- \) to form \( \text{ZnCl}_2 \), which will decrease the concentration of \( \text{Zn}^{2+} \). However, according to Le Chatelier's principle, the reaction will shift to the left to counteract this decrease:
\[ \text{Adding HCl} \rightarrow \text{Shift to left} \rightarrow \text{Increase } [\text{Zn}^{2+}] \]

### Step 5: Consider the Effects of Adding More Ammonia
If we add more ammonia \( \text{NH}_3 \), it will increase the concentration of the reactants. According to Le Chatelier's principle, the reaction will shift to the right to form more complex ions, which will decrease the concentration of \( \text{Zn}^{2+} \):
\[ \text{Adding NH}_3 \rightarrow \text{Shift to right} \rightarrow \text{Decrease } [\text{Zn}^{2+}] \]

### Conclusion
To increase the concentration of \( \text{Zn}^{2+} \), we can either add water or hydrochloric acid. Both actions will shift the equilibrium to the left, increasing the concentration of \( \text{Zn}^{2+} \).

### Final Answer
To increase the concentration of \( \text{Zn}^{2+} \), we have to add **water (H2O)** or **hydrochloric acid (HCl)**.

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