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Given that the heats of combustion of carbon and carbon monoxide are −393.5 kJ mol−1 and −283.5 kJ mol−1, respectively, what is the heat of formation (in kJ) of one mole of carbon monoxide?

1. −676.5
2. −110.5
3. 110.5
4. 676.5

Correct answer: −110.5

Solution

C + O2 -> CO2 is -393.5; CO + 1/2 O2 -> CO2 is -283.5. For C + 1/2 O2 -> CO, dHf = -393.5 - (-283.5) = -110.5 kJ/mol.

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