Gas constant

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Values of R
8.314472 J · K^-1 · mol^-1
0.08205746 L · atm · K^-1 · mol^-1
8.2057459 x 10^-5 m³ · atm · K^-1 · mol^-1
8.314472 L · kPa · K^-1 · mol^-1
62.3637 L · mmHg · K^-1 · mol^-1
62.3637 L · Torr · K^-1 · mol^-1
83.14472 L · mbar · K^-1 · mol^-1
1.987 cal · K^-1 · mol^-1
10.7316 ft³ · psi · °R^-1 · lb · mol^-1

The gas constant (also known as the universal or ideal gas constant, usually denoted by symbol R) is a physical constant used in equations of state to relate various groups of state functions to one another. It is another name for the Boltzmann constant, but when used in the ideal gas law it is usually expressed in the more convenient units of energy per Kelvin per mole rather than simply energy per Kelvin per particle.

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

<math>

p = {RT\over{\tilde{V}}} </math> where p is the pressure of an ideal gas, T is its temperature, and <math>\tilde{V}</math> is its molar volume. R also appears in the Nernst equation as well as in the Lorentz-Lorenz formula. P = power but p = pressure.

Its value is:

R = 8.314472[15] J · K^-1 · mol^-1

or, removing Avogadro's number,

K_B = 1.3806505(24) J · K^-1

The two digits in brackets signify the uncertainty (standard deviation) in the last two digits of the value.

The Boltzmann constant k_B may be used in place of the other forms of the ideal gas constant by working in pure particle count rather than number of moles of gas; this simply requires carrying a factor of Avogadro's number. Writing:

One can then express the ideal gas equation in direct terms of Boltzmann's constant:

<math>\qquad pV=Nk_BT</math>

with N = nN_A is the actual number of molecules.

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