An ideal gas satisfies these conditions:

1. It consists of a large number of identical molecules.
   
   
2. The volume occupied by the molecules themselves is negligible compared to the volume of the container they're in.
   
   
3. The molecules obey Newton's laws of motion, and they move in random motion.
   
   
4. The molecules experience forces only during collisions; any collisions are completely elastic, and take a negligible amount of time.
   
   

The ideal gas law states that:

PV = nRT

where P is pressure, V is volume, n is the number of moles, T is the absolute temperature, and R = 8.31 J/(mol K) is the universal gas constant.

This can be written in terms of N, the number of molecules, instead.

N = nN_A, so:

PV = nN_A (R/N_A) T = NkT

where k is the Boltzmann constant, 1.38 x 10^-23 J/K. This is the universal gas constant divided by Avogadro's number.