|
|
|
|
Gas constant
|
| |
|
| |
The gas constant (also known as the molar, universal, or ideal gas constant, denoted by the symbol or ) 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.
Discussion
Ask a question about 'Gas constant' Start a new discussion about 'Gas constant' Answer questions from other users |
Encyclopedia
Values of R
| Units
(V P T−1 n−1) |
|---|
| 8.314 472(15) | J K−1 mol−1 | | 1.985 8775(34) | cal K−1 mol−1 | | erg K−1 mol−1 | | 8.314 472(15) | m3 Pa K−1 mol−1 | | 8.314 472(15) | cm3 MPa K−1 mol−1 | | m3 bar K−1 mol−1 | | L bar K−1 mol−1 | | 0.082 057 46(14) | L atm K−1 mol−1 | | 62.363 67(11) | L mmHg K−1 mol−1 | | 62.363 67(11) | L Torr K−1 mol−1 | | 6.132 440(10) | ft-lb K−1 g-mol−1 | | 1545.349(3) | ft-lb R−1 lb-mol−1 | | 10.731 59(2) | ft3 psi R−1 lb-mol−1 | | 0.730 2413(12) | ft3 atm R−1 lb-mol−1 | | 998.9701(17) | ft3 mmHg K−1 lb-mol−1 | |
The gas constant (also known as the molar, universal, or ideal gas constant, denoted by the symbol or ) 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, but expressed in units of energy (i.e. the pressure-volume product) per kelvin per mole (rather than energy per kelvin per particle).
Its value is
- R = 8.314 472(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 relative uncertainty is 1.7.
The gas constant occurs in the ideal gas law, as follows:
where p is the absolute pressure, T is thermodynamic temperature, V is the volume of gas, and n is the amount of gas. The gas constant has the same units as molar entropy.
Relationship with the Boltzmann constant
The Boltzmann constant kB (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
where NA is the Avogadro constant.
For example, the ideal gas law in terms of Boltzmann's constant is
.
Specific gas constant
Rspecific
for dry air | Units |
|---|
| 287.06 | J kg−1 K−1 | | 1716.6 | ft-lb slug−1 R−1 | Based on a mean molar mass
for dry air of 28.9645 g/mol. | |
The specific gas constant of a gas or a mixture of gases (Rspecific) is given by the molar gas constant, divided by the molar mass (M) of the gas/mixture.
It is common, especially in engineering applications, to represent the specific gas constant by the symbol . In such cases, the universal gas constant is usually given a different symbol such as to distinguish it. In any case, the context and/or units of the gas constant should make it clear as to whether the universal or specific gas constant is being referred to.
U.S. Standard Atmosphere
The U.S. Standard Atmosphere, 1976 (USSA1976) defines the gas constant R* as:
The USSA1976 does recognize, however, that this value is not consistent with the cited values for the Avogadro constant and the Boltzmann constant. 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 kilometers (the equivalent of a difference of only 0.174 meters or 6.8 inches) and an increase of 0.292 Pa at 20 km (the equivalent of a difference of only 0.338 m or 13.2 in).
See also
|
| |
|
|
