To solve the problem step by step, we will address each part of the question separately.
### Part (i): Calculate the mass of hydrogen peroxide in 2 L of a 5 molar solution.
1. **Understand Molarity**: Molarity (M) is defined as the number of moles of solute per liter of solution. A 5 molar solution means there are 5 moles of hydrogen peroxide (H₂O₂) in 1 liter of solution.
2. **Calculate Moles in 2 L**:
\[
\text{Moles of H₂O₂ in 2 L} = 5 \, \text{moles/L} \times 2 \, \text{L} = 10 \, \text{moles}
\]
3. **Find Molecular Mass of H₂O₂**:
- The molecular mass of H₂O₂ can be calculated as follows:
- Hydrogen (H) has an atomic mass of 1 g/mol, and there are 2 hydrogen atoms: \(2 \times 1 = 2 \, \text{g/mol}\)
- Oxygen (O) has an atomic mass of 16 g/mol, and there are 2 oxygen atoms: \(2 \times 16 = 32 \, \text{g/mol}\)
- Therefore, the molecular mass of H₂O₂ = \(2 + 32 = 34 \, \text{g/mol}\)
4. **Calculate Mass of H₂O₂**:
\[
\text{Mass of H₂O₂} = \text{Number of moles} \times \text{Molecular mass} = 10 \, \text{moles} \times 34 \, \text{g/mol} = 340 \, \text{grams}
\]
### Part (ii): Calculate the mass of oxygen liberated by the decomposition of 200 mL of this solution.
1. **Write the Decomposition Reaction**:
\[
2 \, \text{H₂O₂} \rightarrow 2 \, \text{H₂O} + \text{O₂}
\]
2. **Molar Mass of H₂O₂ and O₂**:
- As calculated earlier, the molar mass of H₂O₂ is 34 g/mol.
- The molar mass of O₂ (oxygen) is \(2 \times 16 = 32 \, \text{g/mol}\).
3. **Determine Moles in 200 mL**:
- Since 200 mL is 0.2 L, we can find the number of moles of H₂O₂ in 200 mL:
\[
\text{Moles of H₂O₂ in 200 mL} = 5 \, \text{moles/L} \times 0.2 \, \text{L} = 1 \, \text{mole}
\]
4. **Calculate Mass of H₂O₂ in 200 mL**:
\[
\text{Mass of H₂O₂} = 1 \, \text{mole} \times 34 \, \text{g/mol} = 34 \, \text{grams}
\]
5. **Use Stoichiometry to Find Mass of O₂**:
- From the reaction, 68 grams of H₂O₂ produces 32 grams of O₂.
- Since we have 34 grams of H₂O₂, we can set up a proportion:
\[
\frac{34 \, \text{grams H₂O₂}}{68 \, \text{grams H₂O₂}} = \frac{x \, \text{grams O₂}}{32 \, \text{grams O₂}}
\]
- Cross-multiplying gives:
\[
x = \frac{34 \times 32}{68} = 16 \, \text{grams of O₂}
\]
### Final Answers:
(i) The mass of hydrogen peroxide in 2 L of a 5 molar solution is **340 grams**.
(ii) The mass of oxygen liberated by the decomposition of 200 mL of this solution is **16 grams**.
