The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law. It can also be derived from kinetic theory, as was achieved (apparently independently) by August Krönig in 1856 and Rudolf Clausius in 1857.
The state of an amount of gas is determined by its pressure, volume, and temperature. The modern form of the equation is:
where p is the absolute pressure of the gas; V is the volume; n is the amount of substance; R is the Regnault constant, better known as universal gas constant; and T is the absolute temperature.
In SI units, p is measured in pascals; V in cubic metres; n in moles; and T in kelvin. R has the value 8.314472 J·K−1·mol−1 in SI units).
The temperature given in the equation of state must be an absolute temperature that begins at absolute zero. In the metric system of units, we must specify the temperature in Kelvin (not degrees Celsius). In the Imperial system, absolute temperature is in Rankine (not degrees Fahrenheit). 
Full article ▸