Determine which substance is the best reducing agent in Q. no. 69 :

Options

A. `HPO_(3)^(2-)`
B. `PO_(4)^(3-)`
C. `IO^(-)`
D. `IO_(3)^(-)`

Answer

To determine which substance is the best reducing agent among the given options, we need to analyze the oxidation states of each substance and their ability to donate electrons. A reducing agent is a substance that loses electrons (is oxidized) and, in doing so, reduces another substance.

### Step-by-Step Solution:

1. **Identify the Oxidation States**:
- For each substance, we need to calculate the oxidation state of the key element (usually the one that can change its oxidation state).

- **HPO3^2-**:
- Let the oxidation state of phosphorus be \( X \).
- The equation becomes: \( X + 3(1) + 3(-2) = -2 \)
- Simplifying gives: \( X - 6 + 3 = -2 \) → \( X - 3 = -2 \) → \( X = +3 \)

- **PO4^3-**:
- Let the oxidation state of phosphorus be \( X \).
- The equation becomes: \( X + 4(-2) = -3 \)
- Simplifying gives: \( X - 8 = -3 \) → \( X = +5 \)

- **IO^-**:
- Let the oxidation state of iodine be \( X \).
- The equation becomes: \( X + (-2) = -1 \)
- Simplifying gives: \( X - 2 = -1 \) → \( X = +1 \)

- **IO3^-**:
- Let the oxidation state of iodine be \( X \).
- The equation becomes: \( X + 3(-2) = -1 \)
- Simplifying gives: \( X - 6 = -1 \) → \( X = +5 \)

2. **Compare the Oxidation States**:
- We have calculated the oxidation states:
- HPO3^2-: +3
- PO4^3-: +5
- IO^-: +1
- IO3^-: +5

3. **Determine the Ability to Donate Electrons**:
- A good reducing agent is one that can easily lose electrons.
- The lower the oxidation state, the more likely the substance is to donate electrons.
- Comparing the oxidation states:
- HPO3^2- (+3) can donate 2 electrons.
- PO4^3- (+5) is less likely to donate electrons.
- IO^- (+1) can donate 1 electron.
- IO3^- (+5) is also less likely to donate electrons.

4. **Conclusion**:
- Among the options, HPO3^2- with an oxidation state of +3 is the best reducing agent because it can donate electrons more readily compared to the others.

### Final Answer:
The best reducing agent among the given options is **HPO3^2-**.