Gas constant
| Values of R |
|---|
| 8.314570 (J)(K-1)(mol-1) |
| 0.0820578 (L)(atm)(K-1)(mol-1) |
| 62.3639 (L)(torr)(K-1)(mol-1) |
| 1.987 (cal)(K-1)(mol-1) |
Molar gas constant (also known as universal gas constant, usually denoted by symbol R) is the constant occurring in the universal gas equation, i.e. the equation of state of an ideal gas:
- <math>p*V = n*R*T<math>
Here p is the pressure of gas, V the volume it ocupies, n the number of moles of gas, and T its temperature.
It can be shown that R is an universal constant, equal for all gases. Real gases obey this equation only in an approximation of very diluted gases.
R also appears in the Nernst equation as well as in the Lorentz-Lorenz formula.
Its value is:
- R = 8.314570[70] J K-1 mol-1
The two digits in brackets signify the uncertainty (standard deviation) in the last two digits of the value.
The Boltzmann constant kB is defined as a ratio of molar gas constant and the Avogadro's number:
- <math>k_B = \frac{R}{N_A}<math>
With it we can write the universal gas equation
- <math>p*V = n*k_B*T<math>
with n now the actual number of molecules.
Example
1 cubic metre of air at a pressure of 100,000 Pa and a temperature of 300 K has a mass of 1.15 kg, and, since air has an average molecular mass of 29, this is 40 moles.
Substituting in the universal gas equation gives: 100,000 x 1 = 40 x 8.3 x 300
More generally, 1 cubic metre of any gas at a pressure of 100,000 Pa and a temperature of 300 K is 40 moles.
