Which of the following statements is not correct ?

Options

A. The oxidation number of `S` in `(NH_(4))_(2)S_(2)O_(8)` is `+6`.
B. The oxidation number of `O` s in `OsO_(4)` is `+8`.
C. The oxidation number of `S` in `H_(2)SO_(5)` is `+8`.
D. The oxidation number of `O` in `KO_(2)` is` -1//2`.

Explanation

To solve the question regarding which of the following statements is not correct, we will analyze each statement about the oxidation states of sulfur and other elements in the given compounds.

### Step-by-Step Solution:

1. **Statement Analysis**:
- We have four statements regarding oxidation numbers, particularly focusing on sulfur and osmium in various compounds.

2. **Statement 1**: "The oxidation number of sulfur in \( \text{NH}_4\text{S}_2\text{O}_8 \) is +6."
- **Calculation**:
- The compound can be broken down into \( \text{NH}_4^+ \) and \( \text{S}_2\text{O}_8^{2-} \).
- In \( \text{S}_2\text{O}_8^{2-} \), there are 2 sulfur atoms and 8 oxygen atoms.
- Assume oxygen has an oxidation state of -2, then:
\[ 2S + 8(-2) = -2 \implies 2S - 16 = -2 \implies 2S = 14 \implies S = +7 \]
- However, the maximum oxidation state of sulfur is +6 due to the presence of peroxide linkages.
- Therefore, the oxidation state of sulfur is indeed +6. This statement is **correct**.

3. **Statement 2**: "The oxidation number of osmium in \( \text{OsO}_4 \) is +8."
- **Calculation**:
- In \( \text{OsO}_4 \), with each oxygen at -2:
\[ Os + 4(-2) = 0 \implies Os - 8 = 0 \implies Os = +8 \]
- This statement is also **correct**.

4. **Statement 3**: "The oxidation number of sulfur in \( \text{H}_2\text{SO}_5 \) is +8."
- **Calculation**:
- The structure of \( \text{H}_2\text{SO}_5 \) includes a sulfur atom bonded to five oxygens and two hydrogens.
- Assuming oxygen as -2:
\[ 2H + S + 5(-2) = 0 \implies 2 + S - 10 = 0 \implies S = +8 \]
- However, sulfur cannot exceed +6. Thus, this statement is **incorrect**.

5. **Statement 4**: "The oxidation number of \( \text{KO}_2 \) is -1/2."
- **Calculation**:
- In \( \text{KO}_2 \), potassium (K) has an oxidation state of +1, and the two oxygens are in a superoxide form.
- The overall charge is neutral:
\[ K + 2O = 0 \implies 1 + 2O = 0 \implies 2O = -1 \implies O = -\frac{1}{2} \]
- This statement is **correct**.

### Conclusion:
The statement that is not correct is **Statement 3**: "The oxidation number of sulfur in \( \text{H}_2\text{SO}_5 \) is +8."