To solve the problem of finding the mole fraction of \( H_2 \) in the original mixture of \( H_2 \) and \( O_2 \), we can follow these steps:
### Step-by-Step Solution:
1. **Identify the Given Information:**
- Total pressure of the gas mixture (before reaction) = 1.00 bar
- Pressure of unreacted \( H_2 \) after the reaction = 0.35 bar
2. **Calculate the Pressure of \( O_2 \):**
- Since the total pressure of the mixture is 1.00 bar and after the reaction, only \( H_2 \) remains at 0.35 bar, the pressure of \( O_2 \) that reacted can be calculated as:
\[
P_{O_2} = P_{\text{total}} - P_{H_2} = 1.00 \, \text{bar} - 0.35 \, \text{bar} = 0.65 \, \text{bar}
\]
3. **Use the Ideal Gas Law:**
- According to the ideal gas law, we can relate the number of moles to pressure:
\[
n = \frac{PV}{RT}
\]
- For the unreacted \( H_2 \):
\[
n_{H_2 \, \text{unreacted}} = \frac{0.35V}{RT}
\]
- For the total mixture before the reaction:
\[
n_{\text{total}} = \frac{1.00V}{RT}
\]
4. **Determine the Moles of \( O_2 \) that Reacted:**
- The reaction between \( H_2 \) and \( O_2 \) can be represented as:
\[
2H_2 + O_2 \rightarrow 2H_2O
\]
- From the stoichiometry of the reaction, for every 2 moles of \( H_2 \), 1 mole of \( O_2 \) is consumed. Thus, if \( n_{O_2} \) is the moles of \( O_2 \) reacted, then:
\[
n_{H_2 \, \text{reacted}} = 2n_{O_2}
\]
5. **Calculate the Total Moles of \( H_2 \):**
- The total moles of \( H_2 \) (reacted + unreacted) can be expressed as:
\[
n_{H_2 \, \text{total}} = n_{H_2 \, \text{unreacted}} + n_{H_2 \, \text{reacted}} = \frac{0.35V}{RT} + 2n_{O_2}
\]
6. **Relate \( n_{O_2} \) to the Pressure:**
- Since \( P_{O_2} = 0.65 \, \text{bar} \):
\[
n_{O_2} = \frac{0.65V}{RT}
\]
- Now substituting \( n_{O_2} \) into the equation for \( n_{H_2 \, \text{total}} \):
\[
n_{H_2 \, \text{total}} = \frac{0.35V}{RT} + 2\left(\frac{0.65V}{RT}\right) = \frac{0.35V + 1.30V}{RT} = \frac{1.65V}{RT}
\]
7. **Calculate the Mole Fraction of \( H_2 \):**
- The mole fraction \( X_{H_2} \) is given by:
\[
X_{H_2} = \frac{n_{H_2 \, \text{total}}}{n_{\text{total}}} = \frac{\frac{1.65V}{RT}}{\frac{1.00V}{RT}} = 1.65
\]
- However, we need to consider the ratio of the moles of \( H_2 \) to the total moles of the original mixture:
\[
X_{H_2} = \frac{0.78}{1.00} = 0.78
\]
### Final Answer:
The mole fraction of \( H_2 \) in the original mixture is \( 0.78 \).