Arrange the acids (I) `H_2SO_3`, (II) `H_3PO_3` and (III) `HClO_3` in the decreasing order of acidity.

Options

A. `I gt III gt IV`
B. `I gt II gt III`
C. `II gt III gt I`
D. `III gt I gt II`

Answer

To arrange the acids \( H_2SO_3 \), \( H_3PO_3 \), and \( HClO_3 \) in the decreasing order of acidity, we can follow these steps:

### Step 1: Determine the oxidation states of the central atoms in each acid.

1. **For \( H_2SO_3 \)**:
- Let the oxidation state of sulfur (S) be \( x \).
- The equation based on the oxidation states is:
\[ 2(+1) + x + 3(-2) = 0 \]
Simplifying this gives:
\[ 2 + x - 6 = 0 \implies x - 4 = 0 \implies x = +4 \]
- Therefore, the oxidation state of sulfur in \( H_2SO_3 \) is +4.

2. **For \( H_3PO_3 \)**:
- Let the oxidation state of phosphorus (P) be \( y \).
- The equation based on the oxidation states is:
\[ 3(+1) + y + 3(-2) = 0 \]
Simplifying this gives:
\[ 3 + y - 6 = 0 \implies y - 3 = 0 \implies y = +3 \]
- Therefore, the oxidation state of phosphorus in \( H_3PO_3 \) is +3.

3. **For \( HClO_3 \)**:
- Let the oxidation state of chlorine (Cl) be \( z \).
- The equation based on the oxidation states is:
\[ (+1) + z + 3(-2) = 0 \]
Simplifying this gives:
\[ 1 + z - 6 = 0 \implies z - 5 = 0 \implies z = +5 \]
- Therefore, the oxidation state of chlorine in \( HClO_3 \) is +5.

### Step 2: Compare the oxidation states to determine acidity.

- The acidity of an acid is generally related to the oxidation state of the central atom. Higher oxidation states typically lead to stronger acids.
- We have determined the oxidation states as follows:
- \( H_2SO_3 \): +4
- \( H_3PO_3 \): +3
- \( HClO_3 \): +5

### Step 3: Arrange the acids in decreasing order of acidity.

- Based on the oxidation states:
- \( HClO_3 \) (Cl with +5) > \( H_2SO_3 \) (S with +4) > \( H_3PO_3 \) (P with +3)

Thus, the order of acidity from strongest to weakest is:
\[ HClO_3 > H_2SO_3 > H_3PO_3 \]

### Final Answer:
The decreasing order of acidity is:
\[ HClO_3 > H_2SO_3 > H_3PO_3 \]