A : `H_(3) PO_4` is less acidic than `H_(3) PO_(3)` .
R : Oxidation state of phosphorus in `H_(3) PO_(4) lt H_(3) PO_(3)` .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion , then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false , then mark (3)
D. If both Assertion and Reason are false statements , then mark (4)

Answer

To solve the question, we need to analyze the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that \( H_3PO_4 \) (phosphoric acid) is less acidic than \( H_3PO_3 \) (phosphorous acid).

- **Understanding Acidity**: The acidity of a compound is often related to the stability of its conjugate base. A more stable conjugate base corresponds to a stronger acid.
- **Structures**:
- **\( H_3PO_4 \)**: Has the structure \( P(=O)(OH)_3 \).
- **\( H_3PO_3 \)**: Has the structure \( P(=O)(OH)_2(OH) \).

### Step 2: Determine the Conjugate Bases
- **Conjugate Base of \( H_3PO_4 \)**: Remove one \( H^+ \) to get \( H_2PO_4^- \).
- **Conjugate Base of \( H_3PO_3 \)**: Remove one \( H^+ \) to get \( H_2PO_3^- \).

### Step 3: Stability of Conjugate Bases
- **Resonance Structures**: The stability of the conjugate bases can be analyzed through resonance.
- **\( H_2PO_4^- \)** has resonance structures that can stabilize the negative charge.
- **\( H_2PO_3^- \)** also has resonance structures, but the presence of an additional \( OH \) group in \( H_3PO_3 \) can lead to better charge distribution.

### Step 4: Conclusion on Acidity
- The assertion is correct: \( H_3PO_4 \) is indeed less acidic than \( H_3PO_3 \) because the conjugate base \( H_2PO_3^- \) is more stable due to better resonance stabilization.

### Step 5: Analyze the Reason
The reason states that the oxidation state of phosphorus in \( H_3PO_4 \) is less than that in \( H_3PO_3 \).

- **Calculate Oxidation States**:
- For \( H_3PO_4 \):
- \( H: +3 \) (3 H atoms)
- \( O: -8 \) (4 O atoms, each -2)
- Let \( X \) be the oxidation state of phosphorus.
- Equation: \( +3 + X - 8 = 0 \) → \( X = +5 \)

- For \( H_3PO_3 \):
- \( H: +3 \) (3 H atoms)
- \( O: -6 \) (3 O atoms, each -2)
- Let \( Y \) be the oxidation state of phosphorus.
- Equation: \( +3 + Y - 6 = 0 \) → \( Y = +3 \)

### Step 6: Conclusion on Reason
- The reason is incorrect: The oxidation state of phosphorus in \( H_3PO_4 \) is +5, which is greater than the oxidation state in \( H_3PO_3 \) (+3).

### Final Conclusion
- The assertion is true, but the reason is false. Therefore, the correct answer is that the assertion is true, and the reason is false.