Cliffs AP Chemistry, 3rd Edition

Real gas behavior deviates from the values obtained using the ideal gas equation because the

ideal gas equation assumes that (1) the molecules do not occupy space and (2) there is no at-

tractive force between the individual molecules. However, at low temperatures (just above the

boiling point of the liquid), these factors become significant, and we must use an alternative

equation, known as the van der Waals equation, that accounts for them.

At the lower temperatures, a greater attraction exists between the molecules, so the compress-

ibility of the gas is significant. This causes the product of P ⋅V to be smaller than predicted.

Because P ⋅V is found in the denominator in the foregoing equation, the calculated molecular

weight would tend to be higher than the molecular weight actually is.

Questions Commonly Asked About the AP Chemistry Exam

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