Final answer:
Hexane, with the largest molecular mass, will have the highest boiling point, followed by pentane, and then propane with the smallest molecular mass will have the lowest boiling point.
Step-by-step explanation:
To rank the boiling points of propane, pentane, and hexane from highest to lowest, we consider the molecular mass and structural effects on the London dispersion forces. Since these compounds are alkanes and nonpolar, the boiling points are determined largely by the strength of the London dispersion forces. These forces increase with molecular mass and surface contact area. Therefore, hexane, with the highest molecular mass and larger surface area, will have the highest boiling point. Pentane comes next, and propane, with the smallest molecular mass, has the lowest boiling point. The order from highest to lowest boiling point is hexane, pentane, then propane.
The four compounds are alkanes and nonpolar, so London dispersion forces are the only important intermolecular forces. These forces are generally stronger with increasing molecular mass, so propane should have the lowest boiling point and n-pentane should have the highest, with the two butane isomers falling in between. Of the two butane isomers, 2-methylpropane is more compact, and n-butane has the more extended shape. Consequently, we expect intermolecular interactions for n-butane to be stronger due to its larger surface area, resulting in a higher boiling point. The overall order is thus as follows, with actual boiling points in parentheses: propane (-42.1°C) < 2-methylpropane (-11.7°C) < n-butane (-0.5°C) < n-pentane (36.1°C).