Both chromate and dichromate ions are in dynamic equilibrium st pH

Options

A. 6
B. 8
C. 4
D. 10

Explanation

To determine the dynamic equilibrium between chromate (\( \text{CrO}_4^{2-} \)) and dichromate (\( \text{Cr}_2\text{O}_7^{2-} \)) ions at a specific pH, we can follow these steps:

### Step 1: Understand the Chemical Reactions
The chromate and dichromate ions are interconvertible in solution depending on the pH. The relevant reactions are:

1. **Dichromate to Chromate**:
\[ \text{Cr}_2\text{O}_7^{2-} + 2 \text{OH}^- \rightleftharpoons 2 \text{CrO}_4^{2-} + \text{H}_2\text{O} \]

2. **Chromate to Dichromate**:
\[ 2 \text{CrO}_4^{2-} + 2 \text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \]

### Step 2: Analyze the Effect of pH
- In a **basic medium** (high pH), the equilibrium shifts towards the formation of chromate ions (\( \text{CrO}_4^{2-} \)).
- In an **acidic medium** (low pH), the equilibrium shifts towards the formation of dichromate ions (\( \text{Cr}_2\text{O}_7^{2-} \)).

### Step 3: Identify the pH at Equilibrium
The dynamic equilibrium between these two forms occurs at a specific pH. The transition point is typically around pH 4. At this pH, both species can coexist in solution, and the reactions can proceed in both directions:

- Above pH 4, the chromate ion predominates.
- Below pH 4, the dichromate ion predominates.

### Step 4: Conclusion
Thus, the dynamic equilibrium between chromate and dichromate ions is established around pH 4.

### Final Answer
The correct answer is that both chromate and dichromate ions are in dynamic equilibrium at **pH 4**.

---