Gas Constant (R) Definition

Chemistry Glossary Definition of Gas Constant (R)

The gas constant R is used in the Ideal Gas Law and Nernst Equation.
The gas constant R is used in the Ideal Gas Law and Nernst Equation. Jeremy Hudson / Getty Images

Chemistry and physics equations commonly include "R", which is the symbol for the gas constant, molar gas constant, or universal gas constant.

Key Takeaways: Gas Constant

  • The gas constant R is a proportionality constant used in the ideal gas law and Nernst equation.
  • Units for the gas constant vary, depending on other units used in the equation. One common value is 8.3145 J/mol·K.
  • The gas constant is essentially the same as the Boltzmann constant, except expressed in a different form.

Gas Constant Definition

The Gas Constant is the physical constant in the equation for the Ideal Gas Law:

  • PV = nRT

It's also found in the Nernst equation relating the reduction potential of a half-cell to the standard electrode potential:

  • E = E0 - (RT/nF)lnQ

where E is the cell potential, E0 is the standard cell potential, R is the gas constant, T is the temperature, n is the number of mole of electrons exchanged, F is Faraday's constant, and Q is the reaction quotient.

The gas constant is equivalent to the Boltzmann constant, just expressed in units of energy per temperature per mole, while the Boltzmann constant is given in terms of energy per temperature per particle. From a physical standpoint, the gas constant is a proportionality constant that related the energy scale to the temperature scale for a mole of particles at a given temperature.

Value of the Gas Constant

The value of the gas constant 'R' depends on the units used for pressure, volume and temperature.

  • R = 0.0821 liter·atm/mol·K
  • R = 8.3145 J/mol·K
  • R = 8.2057 m3·atm/mol·K
  • R = 62.3637 L·Torr/mol·K or L·mmHg/mol·K

Why R Is Used for the Gas Constant

Some people assume the symbol R is used for the gas constant in honor of the French chemist Henri Victor Regnault, who performed experiments that were first used to determine the constant. However, it's unclear whether his name is the true origin of the convention used to denote the constant.

Specific Gas Constant

A related factor is the specific gas constant or individual gas constant. This may be indicated by R or Rgas. It is the universal gas constant divided by the molar mass (M) of a pure gas or mixture. This constant is specific to the particular gas or mixture (hence its name), while the universal gas constant is the same for any ideal gas.