Gas Constant

The gas constant (also known as the molar, universal, or ideal gas constant, denoted by the symbol R or R) is a physical constant which is featured in many fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation. It is equivalent to the Boltzmann constant, but expressed in units of energy (i.e. the pressure-volume product) per temperature increment per mole (rather than energy per temperature increment per particle). The constant is also a combination of the constants from Boyle's Law, Charles's Law, Avogadro's Law, and Gay-Lussac's Law.

Physically, the gas constant is the constant of proportionality that happens to relate the energy scale in physics to the temperature scale, when a mole of particles at the stated temperature is being considered. Thus, the value of the gas constant ultimately derives from historical decisions and accidents in the setting of the energy and temperature scales, plus similar historical setting of the value of the molar scale used for the counting of particles. The last factor is not a consideration in the value of the Boltzmann constant, which does a similar job of equating linear energy and temperature scales.

The gas constant value is

The two digits in parentheses are the uncertainty (standard deviation) in the last two digits of the value. The relative uncertainty is 9.1×10−7. Some have suggested that it might be appropriate to name the symbol R the Regnault constant in honor of the French chemist Henri Victor Regnault, whose accurate experimental data was used to calculate the early value of the constant; however, the exact reason for the original representation of the constant by the letter R is elusive.

The gas constant occurs in the ideal gas law, as follows:

where P is the absolute pressure (SI unit pascals), V is the volume of gas (SI unit cubic metres), n is the chemical amount of gas (SI unit moles), and T is the thermodynamic temperature (SI unit kelvins). The gas constant is expressed in the same physical units as molar entropy and molar heat capacity.