Final answer:
The molecule isolated closest to the top of a fractionating tower at a refinery is typically the one with the lowest boiling point, which here is C8H18 or octane. It has fewer carbon atoms and a smaller molecular size, causing it to be more volatile and condense higher in the tower than larger molecules.
Step-by-step explanation:
The molecule that would probably be isolated closest to the top of a fractionating tower at a refinery is C₈H₁₈, which is octane. In a petroleum distillation tower, hydrocarbons are separated based on their boiling points. The least volatile components with the highest boiling points condense at the bottom of the tower, while the most volatile components with the lowest boiling points condense at the top.
As molecular size increases, boiling points typically increase; thus, smaller molecules rise higher in the tower. Hence, of the given options, C₂₀H₄₂, C₄₀H₈₂, C₈H₁₈, C₄H₁₀, and C₁₀H₂₂, the octane (C₈H₁₈) with lesser carbon atoms and consequently a lower boiling point would be found higher in the tower compared to the others. Other molecules, like neopentane (C5H12), boast even lower boiling points (9.5°C) due to its branched structure and smaller size relative to its isomers.