During the oxidation of arsenite to arsenate ion in alkaline medium, the number of moles of hydroxide ions involved per mole of arsenite ion are

Options

A. 2
B. 3
C. `2//3`
D. `3//2`

Answer

To solve the problem of determining the number of moles of hydroxide ions involved in the oxidation of arsenite (AsO3^3-) to arsenate (AsO4^3-) in alkaline medium, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants and Products**:
- Reactant: Arsenite ion (AsO3^3-)
- Product: Arsenate ion (AsO4^3-)

2. **Write the Half-Reaction**:
- The half-reaction for the oxidation of arsenite to arsenate can be written as:
\[ \text{AsO}_3^{3-} \rightarrow \text{AsO}_4^{3-} \]

3. **Balance the Arsenic Atoms**:
- The arsenic atoms are already balanced (1 on each side).

4. **Balance the Oxygen Atoms**:
- On the left side, there are 3 oxygen atoms (in AsO3^3-), and on the right side, there are 4 oxygen atoms (in AsO4^3-). To balance the oxygen, we need to add 1 water molecule (H2O) to the left side:
\[ \text{AsO}_3^{3-} + \text{H}_2\text{O} \rightarrow \text{AsO}_4^{3-} \]

5. **Balance the Hydrogen Atoms**:
- Now, we have 2 hydrogen atoms on the left (from H2O) and none on the right. To balance the hydrogen, we need to add 2 hydrogen ions (H+) to the right side:
\[ \text{AsO}_3^{3-} + \text{H}_2\text{O} \rightarrow \text{AsO}_4^{3-} + 2\text{H}^+ \]

6. **Convert to Alkaline Medium**:
- Since the reaction occurs in an alkaline medium, we need to neutralize the H+ ions by adding hydroxide ions (OH-). For every H+ ion, we add one OH- ion:
\[ \text{AsO}_3^{3-} + \text{H}_2\text{O} + 2\text{OH}^- \rightarrow \text{AsO}_4^{3-} + 2\text{H}_2\text{O} \]

7. **Simplify the Equation**:
- Now, we can simplify the equation by canceling out the water molecules:
\[ \text{AsO}_3^{3-} + 2\text{OH}^- \rightarrow \text{AsO}_4^{3-} + \text{H}_2\text{O} \]

8. **Determine the Number of Moles of Hydroxide Ions**:
- From the balanced equation, we see that 2 moles of hydroxide ions (OH-) are required to oxidize 1 mole of arsenite (AsO3^3-) to arsenate (AsO4^3-).

### Final Answer:
The number of moles of hydroxide ions involved per mole of arsenite ion is **2 moles**.