Vapor Pressure: Molecular Size - Pentane, Hexane and Heptane
The measurement of pressure exerted by a vapor is demonstrated using barometers. The vapor pressures of pentane, hexane and heptane are compared, showing the effect of molecular size on vapor pressure. The 7 thumbnail images summarize the content of the video. Click an image to see the image gallery.
Pentane, hexane and heptane differ only in the length of their carbon chain, and have the same type of intermolecular forces, namely dispersion forces. Dispersion forces increase with molecular weight.
Pentane, the smallest of the three, is injected (into the open end of the barometer, it rises to the top) and vaporizes. Here, the mercury is depressed from 730 to 285 millimeters. When hexane is injected, the hexane rises to the top and vaporizes. Here, the mercury is depressed to 596 millimeters. When heptane, the biggest of the three, is injected, the heptane rises to the top and vaporizes. Here, the mercury is depressed to only 686 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 vapor pressure.
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