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Given C + O2 = CO2, ΔHf° = −x kJ 2CO2 = 2CO + O2, ΔHf° = y kJ or CO2 = CO + 1/2 O2, ΔHf° = y/2 kJ From eq. no. (a) and (c) C + 1/2 O2 = CO, ΔHf° = y/2 − x

1. (a) y/2 − x
2. (b) y − x
3. (c) y + x
4. (d) y − 2x

Correct answer: (a) y/2 − x

Solution

Using Hess's Law, the enthalpy change for the reaction C + 1/2 O2 → CO can be derived by combining the given equations. Subtracting equation (a) from equation (c) gives ΔHf° = (y/2) − x.

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