Which of the following does not contain the true peroxide ion?

Options

A. `Na_(2)O_(2)`
B. `H_(2)O_(2)`
C. `BaO_(2)`
D. `SrO_(2)`

Answer

To determine which of the given compounds does not contain the true peroxide ion, we need to analyze the oxidation states of oxygen in each compound. The true peroxide ion (O2^2-) has an oxidation state of -1 for each oxygen atom. Let's evaluate each option step by step.

### Step-by-Step Solution:

1. **Identify the Compounds**: The options given are Na2O2, H2O2, BaO2, and SrO2.

2. **Analyze Na2O2**:
- Sodium (Na) has an oxidation state of +1.
- For Na2O2, we have 2 Na atoms contributing +2.
- Let the oxidation state of oxygen be x. Therefore, the equation is:
\[ 2(+1) + 2x = 0 \implies 2 + 2x = 0 \implies 2x = -2 \implies x = -1 \]
- Conclusion: Na2O2 contains the true peroxide ion.

3. **Analyze H2O2**:
- Hydrogen (H) has an oxidation state of +1.
- For H2O2, we have 2 H atoms contributing +2.
- Let the oxidation state of oxygen be x. Therefore, the equation is:
\[ 2(+1) + 2x = 0 \implies 2 + 2x = 0 \implies 2x = -2 \implies x = -1 \]
- Conclusion: H2O2 contains the peroxide ion, but it can also be represented as two hydroxyl groups (OH)2. This indicates that it does not contain the true peroxide ion.

4. **Analyze BaO2**:
- Barium (Ba) has an oxidation state of +2.
- For BaO2, we have 1 Ba atom contributing +2.
- Let the oxidation state of oxygen be x. Therefore, the equation is:
\[ +2 + 2x = 0 \implies 2x = -2 \implies x = -1 \]
- Conclusion: BaO2 contains the true peroxide ion.

5. **Analyze SrO2**:
- Strontium (Sr) has an oxidation state of +2.
- For SrO2, we have 1 Sr atom contributing +2.
- Let the oxidation state of oxygen be x. Therefore, the equation is:
\[ +2 + 2x = 0 \implies 2x = -2 \implies x = -1 \]
- Conclusion: SrO2 contains the true peroxide ion.

6. **Final Conclusion**: Among the given options, H2O2 does not contain the true peroxide ion because it can be represented as hydroxyl groups, while the others (Na2O2, BaO2, and SrO2) contain the true peroxide ion.

### Answer:
The compound that does not contain the true peroxide ion is **H2O2** (option 2).