Gas constant

The gas constant, also called as the universal, denoted by symbol R is a physical constant which is featured in a large number of fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation. It is equivalent to the Boltzmann constant, and can be expressed in units of energy per Kelvin per mole.

The value of it can be given by:

R = 8.314472(15) J ·K ^-1  · mol ^-1

Here the number in parentheses represents the uncertainty in the last 2 digits.

The gas constant occurs in simplest equation of state, ideal gas law.

The gas constant is the constant of proportionality (R) in the equation

                        PV = nRT

Where P is the absolute Pressure, V is the Volume, n is the number of moles, T is the absolute Temperature.

The above equation relates the volume and pressure of a quantity of gas to the absolute temperature. The specific gas constant  of a gas or a mixture of gases ( R ) is given by the universal gas constant, divided by the molar mass ( M) of the gas/mixture.
The gas constant can be defined as the pressure of the gas multiplied by the volume, divided by the number of moles of the gas multiplied by its temperature in Kelvin. Ideal gases are imaginary that is they strictly obey all gas laws and have a molar volume of 22.4141 liters at standard temperature and pressure (STP), which is 273 Kelvin, 1 atmosphere. At STP, although, most gases behave like ideal gases, so the value of R is generally 0.0821 L atm / mol K or 8.3145 J / mol K. At a low temperature or under high pressure conditions the gas molecules are moving closer together and slowly, so intermolecular forces, called as Vander Waals, makes the measured pressure to be lower than expected. When the molecules are closer together, the volume of the actual molecules becomes a higher fraction of the total volume of the gas.