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Narrative
Pentane, hexane and heptane differ in molecular weight due to the differences in the lengths of their carbon chains. They have the same type of intermolecular forces, namely, London dispersion forces, which increase with molecular weight.

Pentane, the smallest of the three, depresses the mercury column from 730 to 283 millimeters. When hexane is injected, the mercury is depressed to 596 millimeters. While heptane, the biggest of the three, depresses the column to only 683 millimeters.

The dispersion forces between the smaller pentane molecules are much weaker than the forces between the larger heptane molecules. Therefore pentane has the largest vapor pressure and heptane has the smallest.