The heat of combustion for $C$ , $H_{2}$ and $C H_{4}$ are $- 349$ kJ/mol, $- 241.8$ kJ/mol and $- 906.7$ kJ/mol respectively. The heat of formation in kJ/mol of $C H_{4}$ is :

174.1

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274.1

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374.1

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74.1

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Solution

The correct option is B $74.1$
Given that,

$C + O_{2} \to C O_{2} (g); Δ H = - 349$ kJ/mol .......... $(i)$
$H_{2} + \frac{1}{2} O_{2} \to H_{2} O (l); Δ H = - 241.8$ kJ/mol .......... $(i i)$
$C H_{4} + 2 O_{2} \to C O_{2} (g) + 2 H_{2} O (l); Δ H = - 906.7$ kJ/mol ......... $(i i i)$
Now, we perform $(i) + 2 \times (i i) - (i i i)$ , we get
$C (s) + 2 H_{2} (g) \to C H_{4} (g)$
Therefore, $Δ H = - 349 + 2 (- 241.8) + 906.7$ $= 74.1$ kJ/mol

Hence, the heat of formation of $C H_{4}$ is $74.1$ kJ/mol.

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