equation of state

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An equation that relates the pressure p, volume V, and thermodynamic temperature T of an amount of substance n. The simplest is the ideal gas law:

pV = nRT,

where R is the universal gas constant. Applying only to ideal gases, this equation takes no account of the volume occupied by the gas molecules (according to this law if the pressure is infinitely great the volume becomes zero), nor dzoes it take into account any forces between molecules. A more accurate equation of state would therefore be

(p+k)(Vnb) = nRT,

where k is a factor that reflects the decreased pressure on the walls of the container as a result of the attractive forces between particles, and nb is the volume occupied by the particles themselves when the pressure is infinitely high. In van der Waals's equation, proposed by the Dutch physicist J. D. van der Waals (1837–1923),

k = n2a/V2,

where a is a constant. This equation more accurately reflects the behaviour of real gases; several others have done better but are more complicated.

Subjects: Chemistry — Physics.

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