Question 1
Multiple choiceMedium1 markA 23.0 g sample of bioethanol, C₂H₅OH, is burned completely. Assume all its carbon was fixed from atmospheric CO₂ by recent photosynthesis, and ignore emissions from production and transport. The combustion equation is C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Use M(C₂H₅OH) = 46.0 g mol⁻¹ and M(CO₂) = 44.0 g mol⁻¹. What are the mass of CO₂ released and the net change in atmospheric CO₂ from fixation and combustion together? Treat an increase as positive.
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Answer: C. The equation gives 2 mol CO₂ per mole of ethanol. The ethanol amount is 23.0/46.0 = 0.500 mol, so combustion releases 1.00 mol CO₂ × 44.0 g mol⁻¹ = 44.0 g. The same 1.00 mol of atmospheric CO₂ had been fixed to provide the carbon in the ethanol, so the net change is +44.0 − 44.0 = 0.0 g.
A: uses a 1:1 rather than 1:2 ethanol-to-CO₂ mole ratio, giving 22.0 g. B: calculates the released CO₂ but omits the earlier photosynthetic uptake. D: counts the uptake as a negative change but omits the CO₂ released on combustion.
Remember: for the stated carbon balance, recently fixed carbon returns to the atmosphere on combustion.