Gas constant

The gas constant (also known as the universal or ideal gas constant, usually denoted by symbol R) is a physical constant which features in 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, but expressed in units of energy per kelvin per mole (rather than energy per kelvin per particle).

Its value is:

R = 8.314472(15) J · K^-1 · mol^-1

The two digits in parentheses are the uncertainty (standard deviation) in the last two digits of the value.

The gas constant occurs in the simplest equation of state, the ideal gas law, as follows:

where

P is the pressure

T is (absolute) temperature

V is the volume the gas occupies

n is the amount of gas (loosely number of moles)

V_m is the molar volume

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Relationship with the Boltzmann constant

The Boltzmann constant k_B (often abbreviated k) may be used in place of the gas constant by working in pure particle count, N, rather than number of moles, n, since R = N_Ak_B, where N_A is Avogadro's number. For example, the ideal gas law in terms of Boltzmann's constant is:

Specific gas constant

The specific gas constant of a gas or a mixture of gases () is given by the universal gas constant, divided by the molar mass (M) of the gas/mixture.

It is common to represent the specific gas constant by the symbol R. In such cases the context and/or units of R should make it clear as to which gas constant is being referred to. For example, the equation for the speed of sound is usually written in terms of the specific gas constant.

The specific gas constant of dry air is

US Standard Atmosphere

The US Standard Atmosphere, 1976 (USSA1976) defines the Universal Gas Constant as:^[1][2]

The USSA1976 does recognize, however, that this value is not consistent with the cited values for the Avogadro constant and the Boltzmann constant.^[2] This disparity is not a significant departure from accuracy, and USSA1976 uses this value of R for all the calculations of the standard atmosphere. When using the ISO value of R, the calculated pressure increases by only 0.62 pascals at 11,000 meters (the equivalent of a difference of only 0.174 meters – or 6.8 inches) and an increase of 0.292 pascals at 20,000 meters (the equivalent of a difference of only 0.338 meters – or 13.2 inches).

See also

• Boltzmann constant

References

1. ^ Standard Atmospheres. Retrieved on 2007-01-07.
2. ^ ^{a b} U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976 (Linked file is 17 MiB).