Identify the compound in which phosphorus exists in the oxidation state of +1.

Options

A. Phosphonic acid `(H_3PO_3)`
B. Phosphinic acid `(H_3PO_2)`
C. Pyrophosphorus acid `(H_4P_2O_5)`
D. Orthophosphoric acid `(H_3PO_4)`

Answer

To solve the question of identifying the compound in which phosphorus exists in the oxidation state of +1, we will analyze each option given in the question step by step.

### Step 1: Analyze Phosphonic Acid (H3PO3)
1. Write the formula: H3PO3.
2. Assign oxidation states:
- Hydrogen (H) = +1 (3 H atoms contribute +3)
- Oxygen (O) = -2 (3 O atoms contribute -6)
3. Set up the equation for the oxidation state of phosphorus (P):
\[ +3 + x - 6 = 0 \]
Simplifying gives:
\[ x - 3 = 0 \implies x = +3 \]
4. Conclusion: Phosphorus in H3PO3 has an oxidation state of +3.

### Step 2: Analyze Phosphinic Acid (H3PO2)
1. Write the formula: H3PO2.
2. Assign oxidation states:
- Hydrogen (H) = +1 (3 H atoms contribute +3)
- Oxygen (O) = -2 (2 O atoms contribute -4)
3. Set up the equation for the oxidation state of phosphorus (P):
\[ +3 + x - 4 = 0 \]
Simplifying gives:
\[ x - 1 = 0 \implies x = +1 \]
4. Conclusion: Phosphorus in H3PO2 has an oxidation state of +1. This is our candidate for the correct answer.

### Step 3: Analyze Pyrophosphoric Acid (H4P2O5)
1. Write the formula: H4P2O5.
2. Assign oxidation states:
- Hydrogen (H) = +1 (4 H atoms contribute +4)
- Oxygen (O) = -2 (5 O atoms contribute -10)
3. Set up the equation for the oxidation state of phosphorus (P):
\[ +4 + 2x - 10 = 0 \]
Simplifying gives:
\[ 2x - 6 = 0 \implies 2x = 6 \implies x = +3 \]
4. Conclusion: Phosphorus in H4P2O5 has an oxidation state of +3.

### Step 4: Analyze Ortho Phosphoric Acid (H3PO4)
1. Write the formula: H3PO4.
2. Assign oxidation states:
- Hydrogen (H) = +1 (3 H atoms contribute +3)
- Oxygen (O) = -2 (4 O atoms contribute -8)
3. Set up the equation for the oxidation state of phosphorus (P):
\[ +3 + x - 8 = 0 \]
Simplifying gives:
\[ x - 5 = 0 \implies x = +5 \]
4. Conclusion: Phosphorus in H3PO4 has an oxidation state of +5.

### Final Conclusion
After analyzing all the compounds, we find that phosphorus exists in the oxidation state of +1 in phosphinic acid (H3PO2).

### Answer:
The compound in which phosphorus exists in the oxidation state of +1 is **Phosphinic Acid (H3PO2)**.