`Cr(OH)_(3)+ClO^(-)+3OH^(-)rarr?+Cl^(-)+3H_(2)O`. The missing ion is

Options

A. `Cr_(2)O_(7)^(2-)`
B. `Cr^(3+)`
C. `CrO_(4)^(2-)`
D. `Cr_(2)O_(3)`

Explanation

To solve the question, we need to identify the missing ion in the reaction:

\[ \text{Cr(OH)}_3 + \text{ClO}^- + 3 \text{OH}^- \rightarrow ? + \text{Cl}^- + 3 \text{H}_2\text{O} \]

### Step-by-Step Solution:

1. **Identify the Reactants and Products**:
- Reactants: Chromium hydroxide \(\text{Cr(OH)}_3\), hypochlorite ion \(\text{ClO}^-\), and hydroxide ions \(\text{OH}^-\).
- Products: Chloride ion \(\text{Cl}^-\) and water \(\text{H}_2\text{O}\).
- We need to find the missing product.

2. **Determine the Oxidation States**:
- In \(\text{Cr(OH)}_3\), chromium (Cr) has an oxidation state of +3.
- In \(\text{ClO}^-\), chlorine (Cl) has an oxidation state of +1 (since oxygen is -2).
- The chloride ion \(\text{Cl}^-\) has an oxidation state of -1.

3. **Identify the Redox Reaction**:
- The hypochlorite ion (\(\text{ClO}^-\)) is reduced to chloride ion (\(\text{Cl}^-\)), indicating that chlorine is undergoing reduction.
- Chromium is oxidized from +3 in \(\text{Cr(OH)}_3\) to a higher oxidation state.

4. **Determine the Oxidation State of Chromium in the Product**:
- The stable oxidation state of chromium is +6.
- Therefore, chromium must be oxidized to \(\text{CrO}_4^{2-}\) (chromate ion), which has chromium in the +6 oxidation state.

5. **Write the Complete Reaction**:
- The missing ion in the reaction is \(\text{CrO}_4^{2-}\).
- Thus, the complete balanced reaction is:
\[ \text{Cr(OH)}_3 + \text{ClO}^- + 3 \text{OH}^- \rightarrow \text{CrO}_4^{2-} + \text{Cl}^- + 3 \text{H}_2\text{O} \]

### Final Answer:
The missing ion is \(\text{CrO}_4^{2-}\).