A student hypothesizes that using ethanol-blended gasoline requires a different stoichiometric air–fuel ratio than pure gasoline. Which consideration supports this claim?
Oxygen content in fuel blend
The stoichiometric air-fuel ratio is the exact mass of air needed to burn a fuel completely, and it is fixed by the fuel's chemical composition, especially how much of the oxygen for combustion the fuel already carries.
Pure gasoline is essentially a hydrocarbon and contains no oxygen, so all the oxygen for burning must come from the intake air.
Ethanol, however, is an oxygenated molecule; each ethanol molecule contains an oxygen atom, so a blend brings some of its own oxygen to the reaction.
Because part of the required oxygen is supplied internally, ethanol-blended fuel needs less air per unit of fuel for complete combustion, which shifts the stoichiometric ratio and supports the student's claim.
The octane rating measures knock resistance, not the air demand for complete burning. The evaporation rate affects cold starting and mixture formation, not the chemical stoichiometry. The colour of the blend is a physical appearance with no effect on combustion chemistry.
The consideration that actually changes the required ratio is the fuel's own oxygen content.
Hence, the answer is Oxygen content in fuel blend.
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