Consider the following data
Heat of combustion of $H_{2 (g)} = - 241.8 K J / m o l$
Heat of combustion of $C_{(s)} = - 393.5 K J / m o l$
Heat of combustion of $C_{2} H_{5} O H_{(l)} = - 1234.7 K J / m o l$
The heat of formation of $C_{2} H_{5} O H_{(l)}$ is $- x K J / m o l$ (nearest integer) find the magnitude of ‘x’.

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answer is 278.

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Detailed Solution

$C_{2} H_{5} O H_{(l)} + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O Δ H_{C} = [2 Δ {H^{°}}_{\int} (C O_{2}) + 3 Δ {H^{°}}_{\int} (H_{2} O)] - [Δ {H^{°}}_{\int} {(C_{2} H_{5} O H)}_{(_{l})}] - 1234.7 = [2 \times (- 393.5) + 3 (- 241.8)] - [Δ {H^{°}}_{\int} {(C_{2} H_{5} O H)}_{(_{l})}] [Δ {H^{°}}_{\int} {(C_{2} H_{5} O H)}_{(_{l})}] = - 277.7 K J / m o l ≃ - 278 K J / m o l$