Consider the following data:
- Heat of formation of \( CO_2(g) \) = -393.5 kJ mol\(^{-1}\)
- Heat of formation of \( H_2O(l) \) = -286.0 kJ mol\(^{-1}\)
- Heat of combustion of benzene = -3267.0 kJ mol\(^{-1}\)
The heat of formation of benzene is ________________________ kJ mol\(^{-1}\) (Nearest integer).
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Correct Answer: 49
Solution and Explanation
The combustion reaction of benzene: \[ C_6H_6 + \frac{15}{2} O_2 \rightarrow 6CO_2 + 3H_2O \] Using Hess's law: \[ \Delta H_f(C_6H_6) = \Delta H_c - \left( 6\Delta H_f(CO_2) + 3\Delta H_f(H_2O) \right) \] \[ = -3267 - \left(6(-393.5) + 3(-286.0)\right) \] \[ = -3267 + 2361 + 858 \] \[ = -48.5 \approx 49 \, { kJ/mol} \]
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