Phosphorus forms a number of oxoacids which differ in their structures and oxidation state of phosphorus. All the acids contain phosphorus atom//atoms linked tetrahedrally to four other atoms or groups. Each of them has at least one `P=O` or `P rarr O` unit and one `P-OH` unit. The `OH` group is ionisable but `H` atom linked directly to `P` is non-ionisabl. Structures of all the acids are considered to be derived either from phosphorus acid or phosphoric acid.
Which one has `+ 3` oxidation state ?

Options

A. `H_3 PO_4`
B. `H_3 PO_3`
C. `H_4 P_2 O_7`
D. `H_4 P_2 O_6`

Explanation

To determine which oxoacid of phosphorus has a +3 oxidation state for phosphorus, we will analyze the oxidation states of phosphorus in the given oxoacids. The oxoacids we will consider are H₃PO₄, H₃PO₃, H₄P₂O₇, and H₄P₂O₆.

### Step 1: Analyze H₃PO₄ (Phosphoric Acid)
1. Assign the oxidation state of phosphorus as \( x \).
2. Oxygen typically has an oxidation state of -2. There are 4 oxygen atoms, contributing a total of \( 4 \times (-2) = -8 \).
3. Hydrogen has an oxidation state of +1. There are 3 hydrogen atoms, contributing a total of \( 3 \times (+1) = +3 \).
4. The overall charge of the molecule is 0.

Setting up the equation:
\[ x - 8 + 3 = 0 \]
\[ x - 5 = 0 \]
\[ x = +5 \]

### Step 2: Analyze H₃PO₃ (Phosphorous Acid)
1. Assign the oxidation state of phosphorus as \( x \).
2. There are 3 oxygen atoms, contributing \( 3 \times (-2) = -6 \).
3. There are 3 hydrogen atoms, contributing \( 3 \times (+1) = +3 \).
4. The overall charge is 0.

Setting up the equation:
\[ x - 6 + 3 = 0 \]
\[ x - 3 = 0 \]
\[ x = +3 \]

### Step 3: Analyze H₄P₂O₇ (Pyrophosphoric Acid)
1. Assign the oxidation state of phosphorus as \( x \).
2. There are 7 oxygen atoms, contributing \( 7 \times (-2) = -14 \).
3. There are 4 hydrogen atoms, contributing \( 4 \times (+1) = +4 \).
4. The overall charge is 0.

Setting up the equation:
\[ 2x - 14 + 4 = 0 \]
\[ 2x - 10 = 0 \]
\[ 2x = 10 \]
\[ x = +5 \]

### Step 4: Analyze H₄P₂O₆ (Another Phosphoric Acid)
1. Assign the oxidation state of phosphorus as \( x \).
2. There are 6 oxygen atoms, contributing \( 6 \times (-2) = -12 \).
3. There are 4 hydrogen atoms, contributing \( 4 \times (+1) = +4 \).
4. The overall charge is 0.

Setting up the equation:
\[ 2x - 12 + 4 = 0 \]
\[ 2x - 8 = 0 \]
\[ 2x = 8 \]
\[ x = +4 \]

### Conclusion
From the calculations, we find:
- H₃PO₄ has phosphorus in the +5 oxidation state.
- H₃PO₃ has phosphorus in the +3 oxidation state.
- H₄P₂O₇ has phosphorus in the +5 oxidation state.
- H₄P₂O₆ has phosphorus in the +4 oxidation state.

Thus, the oxoacid with phosphorus in the +3 oxidation state is **H₃PO₃**.