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C(s) + O2(g) → CO2(g) ΔH = −94 kcal/mole H2(g) + 1/2 O2 → H2O(g) ΔH = −68 kcal/mole CH4 + 2O2 → CO2 + 2H2O ΔH = −213 kcal/mole C(s) + 2H2 → CH4(g), ΔH = ?

1. (a) −94 − 136 ± 213 = −17 kcal/mole
2. (b) −94 − 136 ± 213 = −17 kcal/mole
3. (c) −94 − 136 ± 213 = −17 kcal/mole
4. (d) −94 − 136 ± 213 = −17 kcal/mole

Correct answer: (a) −94 − 136 ± 213 = −17 kcal/mole

Solution

Using Hess's Law, the enthalpy change for the formation of CH4 can be calculated by combining the given reactions. The correct calculation gives ΔH = −94 − 136 + 213 = −17 kcal/mole.

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