A law relating the pressure, temperature, and volume of an ideal gas. Many common gases exhibit behavior very close to that of an ideal gas at ambient temperature and pressure. The ideal gas law was originally derived from the experimentally measured Charles' law and Boyle's law. Let P be the pressure of a gas, V the volume it occupies, and T its temperature (which must be in absolute temperature units, i.e., in Kelvin). Then the ideal gas law states PV=nRT (1) where n is the number of moles of gas present and R is the universal gas constant, or equivalently PV=NkT (2) where N is the number of atoms of gas present and k is Boltzmann's constant. Boltzmann's constant and the universal gas constant are related by R=NAk (3) where NA is Avogadro's number. Equation (1) can be written in terms of the mass density ρ molar mass m of the gas as P=ρ?R?T/m (4) since the number of moles of gas per unit volume is given by ρ/m=n/V (5)