Chemistry constants
These are the fixed values that turn up in chemistry calculations. The four you use most: the Avogadro constant NA = 6.022 × 10²³ mol⁻¹ (particles per mole), the molar gas constant R = 8.314 J mol⁻¹ K⁻¹ (in the ideal gas law), the Faraday constant F = 96485 C mol⁻¹ (charge per mole of electrons), and the molar volume of a gas, 22.7 L mol⁻¹ at STP. Values below follow the 2019 SI redefinition; those marked exact have no uncertainty.
The four you use most
Avogadro constant — NA
The number of particles in one mole. Use it to convert between a number of atoms or molecules and an amount in moles.
Molar gas constant — R
Appears in the ideal gas law and in thermodynamics. Pick the form whose units match your data — use the J version with pressure in pascals and volume in m³; use the L·atm version with pressure in atm and volume in litres.
Faraday constant — F
The charge carried by one mole of electrons. In electrolysis it links the charge you pass (current × time) to the moles of substance produced at an electrode.
Molar volume of an ideal gas
The volume one mole of any ideal gas occupies. The older "22.4 L" is for 0 °C and 1 atm and is still common in textbooks — check which standard your course uses.
Full reference table
| Constant | Symbol | Value | Notes |
|---|
Chemistry constants — FAQ
What is the value of Avogadro's number?
6.02214076 × 10²³ mol⁻¹. Since the 2019 SI redefinition it is an exact value — the mole is now defined as this many particles.
What is the value of the gas constant R?
8.314462618 J mol⁻¹ K⁻¹, equivalently 0.082057 L·atm·mol⁻¹·K⁻¹ or 1.9872 cal·mol⁻¹·K⁻¹. Use the one whose units match your pressure and volume.
What is the molar volume of a gas at STP?
22.7 L mol⁻¹ at the IUPAC STP of 0 °C and 100 kPa. The older 22.4 L mol⁻¹ is for 0 °C and 1 atm. At SATP (25 °C, 100 kPa) it is 24.8 L mol⁻¹.
What is standard temperature and pressure?
IUPAC STP is 273.15 K (0 °C) and 100 kPa (1 bar). Many textbooks still use 1 atm (101.325 kPa) for the pressure, which changes the molar volume slightly.
What is the Faraday constant for?
It is the charge on one mole of electrons, 96485 C mol⁻¹. Electrochemistry uses it to connect the charge passed in electrolysis to the amount of substance released.
Why did some constants become "exact" in 2019?
The SI base units were redefined by fixing the numerical values of seven constants (including the Planck constant, elementary charge, Boltzmann constant and Avogadro constant). Those now have no measurement uncertainty.