Chemistry Definition of Gas Constant (R)

The Ideal Gas Constant

Contrasting Creative Red and Blue Liquid Smoke Merging

Hero Images / Getty Images

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

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

  • PV = nRT

P is pressure, V is volume, n is the number of moles, and T is temperature.

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

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.

Units for the gas constant vary, depending on other units used in the equation.

One common value is 8.3145 J/mol·K.

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 an ideal gas.