Alkane Formulas and Boiling Point Trends

Steps:

• Identify octane as an 8-carbon alkane, so its formula is C₈H₁₈ using the general alkane formula CₙH₂ₙ₊₂.
• Confirm butane as a 4-carbon alkane with formula C₄H₁₀.
• Recall that boiling points of alkanes increase with molecular size due to stronger London dispersion forces from larger surface area.
• Conclude octane (longer chain) has a higher boiling point than butane.

Why D is correct:

• D matches octane's formula C₈H₁₈ and the trend where larger alkanes have higher boiling points per van der Waals forces.

Why the others are wrong:

• A: Lists correct formula but incorrectly states lower boiling point, ignoring size-dependent intermolecular forces.
• B: Gives butane's formula and wrongly claims higher boiling point for octane.
• C: Gives butane's formula, failing to identify octane.

Final answer: D