Pentane, Hexane and Heptane
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Pentane, hexane and heptane are injected into separate barometers and their vapor pressures are measured. Molecular models of pentane, hexane and heptane are shown.

Methanol and Ethanol
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Methanol and ethanol are injected into separate barometers and their vapor pressures are measured. Molecular models of methanol and ethanol are shown.

Discussion
Vapor pressure depends on the attraction between the molecules of a liquid. The magnitude of London dispersion forces depends on the polarizability of a molecule, that is, the ease with which the molecule's electron cloud can be distorted. The bigger the molecule, and the more electrons it has, the greater the London forces and the lower the vapor pressure. Bigger molecules usually have higher molecular weights. Thus vapor pressure and molecular weight are inversely related for molecules having the same types of intermolecular forces; the higher the molecular weight, the lower the vapor pressure.