At pH = 11 , `Cr_2O_7^(2-)` ion changes to

Options

A. `CrO_3`
B. `CrO_4^(2-)`
C. `Cr^(3+)`
D. `CrO_2^(2+)`

Explanation

To solve the question regarding the change of the dichromate ion \((Cr_2O_7^{2-})\) at pH = 11, we can follow these steps:

### Step 1: Understand the pH context
At pH = 11, the solution is basic. This means that there are hydroxide ions \((OH^{-})\) present in the solution.

**Hint:** Remember that pH values above 7 indicate a basic solution.

### Step 2: Identify the species involved
In a basic medium, the dichromate ion \((Cr_2O_7^{2-})\) can undergo a transformation. The relevant ions here are:
- Dichromate ion: \((Cr_2O_7^{2-})\)
- Chromate ion: \((CrO_4^{2-})\)

**Hint:** Look for the common transformations of dichromate and chromate ions in different pH conditions.

### Step 3: Write the chemical equation
The transformation of dichromate to chromate in a basic medium can be represented by the following chemical equation:

\[ Cr_2O_7^{2-} + 2 OH^{-} \rightarrow 2 CrO_4^{2-} + H_2O \]

**Hint:** Balancing the equation is crucial; ensure that the number of atoms on both sides is equal.

### Step 4: Identify the color change
In addition to the chemical transformation, it is important to note the color change that occurs. The dichromate ion \((Cr_2O_7^{2-})\) is typically orange, while the chromate ion \((CrO_4^{2-})\) is yellow.

**Hint:** Color changes can help in identifying the ions present in a solution.

### Step 5: Conclusion
At pH = 11, the dichromate ion \((Cr_2O_7^{2-})\) changes to the chromate ion \((CrO_4^{2-})\).

**Final Answer:** \((CrO_4^{2-})\)