Gas Constant
Definition and meaning of Gas Constant in chemistry.
The gas constant, symbol R, links pressure, volume, temperature, and moles of gas in the ideal gas law, PV = nRT. It sets the scale for how much a mole of gas expands or exerts pressure as temperature changes. Its numeric value depends on which units are chosen for pressure and volume.
In more detail
The gas constant equals Avogadro's number multiplied by the Boltzmann constant, written R = NA x kB. This scales the Boltzmann constant, which describes energy per single particle, up to a per-mole basis that chemists find more convenient. Since the 2019 redefinition of the SI system, both Avogadro's number and the Boltzmann constant are exact defined values.
This makes R an exact number in SI units, rather than a measured quantity with experimental uncertainty. Pressure and volume can be expressed in several unit systems, such as atmospheres, pascals, or torr. Because of this, R takes on different numerical values depending on the units chosen.
The physical quantity it represents never changes, only its numerical expression does. Students often mix up which value of R to use in a calculation. Using R = 8.314 J/(mol K) requires pressure in pascals and volume in cubic meters.
Using R = 0.08206 L atm/(mol K) requires pressure in atmospheres and volume in liters. Picking the wrong version of R is one of the most common stoichiometry mistakes students make. The gas constant also appears in other equations besides the ideal gas law. These include the van der Waals equation for real gases and the Arrhenius equation for reaction rates.
Key facts
| Symbol | R |
|---|---|
| SI value | 8.314 J/(mol K) |
| Alternate value | 0.08206 L atm/(mol K) |
| Defined as | R = NA x kB (Avogadro constant x Boltzmann constant) |
| Status since 2019 | Exact defined value in SI units |
| Field | Physical Chemistry |
Consider 1.00 mole of an ideal gas at standard temperature and pressure, 0 degrees Celsius and 1 atm. The ideal gas law gives V = nRT/P. Using R = 0.08206 L atm/(mol K), this works out to about 22.4 liters, the familiar molar volume of a gas at STP.
Frequently asked questions
What is the value of the gas constant in different units?
Common values include 8.314 J/(mol K), 0.08206 L atm/(mol K), and 62.36 L torr/(mol K). All represent the same physical constant expressed with different pressure and volume units.
How is the gas constant related to the Boltzmann constant?
R equals Avogadro's number multiplied by the Boltzmann constant. This converts the per-particle Boltzmann constant into a per-mole quantity useful in chemistry.
Why do students get the wrong answer using the gas constant?
The most common mistake is pairing the wrong value of R with the wrong units for pressure or volume. Always match R's units to the units given in the problem before calculating.