Assumptions behind the Ideal Gas Law

21 PROPERTIES OF IDEAL GASES 7.2.1 Assumptions Behind the Ideal Gas Law 

The simplest treatment of the properties of gases starts with the following assumptions, which will determine the limits of validity of the ideal gas equation P V = nRT  

Gases are mostly empty space at normal pressures. At standard temperature and pressure (STP 1 atm pressure, temperature 273K), the same amount of matter will occupy about 1000 times more volume if it is in the gaseous state than if it is a solid or liquid. At much higher densities (for example, pressures of several hundred atmospheres at 273K) this assumption will not be valid. 

Intermolecular forces between gas molecules are assumed to be negligible, and collisions between gas molecules are ignored (in our initial treatment). Collisions with the container are assumed to be elastic, meaning that both momentum and energy are conserved, as described in Section 7.1 above. 

Gas molecules are constantly moving, with a random distribution of directions and speeds. This is also a very reasonable assumption, unless the molecules in the gas were prepared in some way which (for example) made all of them move at nearly the same speed—and even then, collisions with the walls would randomize the distribution in practice. This distribution of speeds will be found using the Boltzmann distribution we discussed in Chapter 4. 

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