Effects of Extreme Conditions on Gas Behavior

At moderately high pressure, values of PV/RT lower than ideal (less than 1) are due predominantly to intermolecular attractions. 

Moderately high Pea-molecules close enough to Interact 

At ordinary pressures, the voiume is large and gas molecules are too far apart to experience significant attractions. At moderately high external pressures, the volume decreases enough for the molecules to influence each other. As the close-up shows, a gas molecule approaching the container wall experiences intermolecular attractions from neighboring molecules that reduce the force of its impact. As a result, real gases exert less pressure than the ideal gas law predicts. 

Very high Pea-free volume container volume 

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