Final answer:
Hexane has a higher vapor pressure than nonane due to its smaller molecular size and lower mass, leading to weaker London dispersion forces and therefore requiring less energy for its molecules to escape into the vapor phase.
Step-by-step explanation:
The student asked which substance has a higher vapor pressure, hexane or nonane. Based on their molecular structures, both hexane and nonane are nonpolar hydrocarbons, and the intermolecular forces present are primarily London dispersion forces. According to trends in physical properties of alkanes, as molecular weight increases, the boiling point of the substance also increases due to stronger London dispersion forces. Therefore, molecules with larger mass and size typically have lower vapor pressures, because more energy is required for the molecules to escape into the vapor phase.
As such, hexane (C6H14), which is a smaller molecule compared to nonane (C9H20), will have a higher vapor pressure. The correct answer to the student's question is A) Hexane.