Gas constant
R = 8.314 J/(mol·K) — the single proportionality constant that makes PV = nRT the same equation for every gas.
Definition
The universal (molar) gas constant R = 8.314462618 J/(mol·K) is the constant of proportionality in the ideal gas law PV = nRT. Its value is the same for every gas in the dilute limit, which is precisely what makes the ideal gas law universal rather than substance-specific.
R is the macroscopic counterpart of the Boltzmann constant: R = N_A k_B, the energy per mole per kelvin against k_B's energy per molecule per kelvin. It appears far beyond gases — in the van 't Hoff equation for osmotic pressure, the Nernst equation in electrochemistry, the Arrhenius rate law, and anywhere a thermal energy scale per mole is needed.
Since the 2019 SI redefinition R is exact, derived from the exact defined values of the Boltzmann and Avogadro constants. Numerically it can be remembered as roughly 8.31 J/(mol·K), or about 0.0821 L·atm/(mol·K) in the units chemists often use.
History
Emerged as gas measurements were combined into PV = nRT during the nineteenth century; tied to the Boltzmann constant once the molecular interpretation of temperature was established, and fixed exactly in the 2019 SI.