Consider a gas which is confined to the inside of a container of volume V. The gas is found to have a pressure p and a temperature T (in Kelvin!). The gas contains n moles of gas particles. The gas is said to be an ideal gas if its thermodynamic parameters (p, T, n and V ) satisfy the so-called ideal gas equation of state:
pV = nRT (Ideal gas)
where R is called the Universal Gas Constant and has the value R = 8.31 J/(mol K). (These units are read “joules per mole Kelvin”. They result from the fact that 1 Pa times 1 m3 equals 1 J !)
Not all gases behave as ideal gases. (Monatomic gases are the closest that we get to an ideal gas.) Nevertheless, we can often get good approximations to some fundamental results by assuming that the gas under consideration is an ideal gas.