In the thermal decomposition of `C_(2)H_(5)Br` to `C_(2)H_(4)` and HBr , the pressure changed from an initial value of 200 mmHg to 390 mmHg at the end of the reaction .What fraction remained unchanged when the pssure of the mixture was 300 mmHg?

Explanation

To solve the problem, we need to analyze the thermal decomposition of \( C_2H_5Br \) into \( C_2H_4 \) and \( HBr \) in terms of pressure changes and the fraction of the reactant that has decomposed.

### Step-by-Step Solution:

1. **Identify Initial and Final Pressures**:
- Initial pressure (\( P_i \)) = 200 mmHg
- Final pressure when the reaction is complete (\( P_f \)) = 390 mmHg

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