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 determine which statement is not correct regarding oxidation numbers, we will analyze each option step by step.

### Step 1: Analyze the first statement
**Statement:** The oxidation number of sulfur in \( \text{NH}_4^{2+} \text{S}_2\text{O}_8 \) is +6.

**Calculation:**
1. The charge of \( \text{NH}_4^+ \) is +1. Since there are two ammonium ions, the total charge contributed by ammonium is \( 2 \times +1 = +2 \).
2. Let the oxidation number of sulfur be \( x \). The compound has 8 oxygen atoms, each with an oxidation number of -2.
3. The equation for the total charge is:
\[ 2x + 8(-2) = 0 \]
Simplifying this gives:
\[ 2x - 16 = 0 \implies 2x = 16 \implies x = +8 \]
4. However, sulfur can only exhibit a maximum oxidation state of +6. Therefore, this statement is **incorrect**.

### Step 2: Analyze the second statement
**Statement:** The oxidation number of oxygen in \( \text{OSO}_4 \) is +8.

**Calculation:**
1. Let the oxidation number of sulfur in \( \text{OSO}_4 \) be \( x \).
2. The equation is:
\[ x + 4(-2) = 0 \]
Simplifying gives:
\[ x - 8 = 0 \implies x = +8 \]
3. This statement is **correct**.

### Step 3: Analyze the third statement
**Statement:** The oxidation number of sulfur in \( \text{H}_2\text{SO}_5 \) is +8.

**Calculation:**
1. Let the oxidation number of sulfur in \( \text{H}_2\text{SO}_5 \) be \( x \).
2. The equation is:
\[ 2(+1) + x + 5(-2) = 0 \]
Simplifying gives:
\[ 2 + x - 10 = 0 \implies x - 8 = 0 \implies x = +8 \]
3. Again, sulfur can only exhibit a maximum oxidation state of +6. Therefore, this statement is **incorrect**.

### Step 4: Analyze the fourth statement
**Statement:** The oxidation number of oxygen in \( \text{KO}_2 \) is -1/2.

**Calculation:**
1. Potassium has an oxidation number of +1.
2. Let the oxidation number of oxygen be \( x \). The equation is:
\[ +1 + 2x = 0 \]
Simplifying gives:
\[ 2x = -1 \implies x = -\frac{1}{2} \]
3. This statement is **correct**.

### Conclusion
The statement that is not correct is the third statement regarding the oxidation number of sulfur in \( \text{H}_2\text{SO}_5 \), which is claimed to be +8.