Physical constants for chemistry
The constants you need in chemistry, from Avogadro’s number and the gas constant to the masses of the proton, neutron and electron, with their symbols, values and units.
| Constant | Symbol | Value | Unit |
|---|---|---|---|
| Avogadro constant exact | NA | 6.022 140 76 × 10²³ | mol⁻¹ |
| Molar gas constant exact | R | 8.314 462 618… | J/(mol·K) |
| Molar gas constant (L·atm units) exact | R | 0.082 057 366… | L·atm/(mol·K) |
| Faraday constant exact | F | 96 485.332 12… | C/mol |
| Boltzmann constant exact | kB | 1.380 649 × 10⁻²³ | J/K |
| Planck constant exact | h | 6.626 070 15 × 10⁻³⁴ | J·s |
| Elementary charge exact | e | 1.602 176 634 × 10⁻¹⁹ | C |
| Speed of light in vacuum exact | c | 299 792 458 | m/s |
| Atomic mass constant (1 dalton) | u | 1.660 539 1 × 10⁻²⁷ | kg |
| Electron mass | me | 9.109 383 7 × 10⁻³¹ | kg |
| Proton mass | mp | 1.672 621 9 × 10⁻²⁷ | kg |
| Neutron mass | mn | 1.674 927 5 × 10⁻²⁷ | kg |
| Standard atmosphere exact | atm | 101 325 | Pa |
| Molar volume of an ideal gas at 0 °C and 1 atm exact | Vm | 22.413 970 | L/mol |
| Molar volume of an ideal gas at 0 °C and 100 kPa exact | Vm | 22.710 955 | L/mol |
| Molar volume of an ideal gas at 25 °C and 100 kPa exact | Vm | 24.789 570 | L/mol |
| Absolute zero exact | T0 | 0 K = −273.15 | °C |
| Ion product of water | Kw | 1.0 × 10⁻¹⁴ | (25 °C) |
| Rydberg constant | R∞ | 10 973 731.57 | m⁻¹ |
| Bohr radius | a0 | 5.291 772 1 × 10⁻¹¹ | m |
| Vacuum permittivity | ε0 | 8.854 187 8 × 10⁻¹² | F/m |
Exact and measured values
Since 2019, the SI units have been defined by fixing the values of seven constants, including the Avogadro constant, the Planck constant, the elementary charge and the Boltzmann constant. These values are exact by definition, and so are constants calculated from them, such as R = N_A × k and F = N_A × e.
The masses of the electron, proton and neutron are still measured, so their values carry a small uncertainty. They are shown here rounded to eight significant figures.
Which gas constant to use
Use R = 8.314 J/(mol·K) when pressure is in pascals and volume in cubic meters, or pressure in kPa and volume in liters. Use R = 0.08206 L·atm/(mol·K) when pressure is in atmospheres and volume in liters. Both are the same constant in different units.
Molar volume and standard conditions
One mole of an ideal gas takes up 22.41 L at 0 °C and 1 atm, the older definition of STP, and 22.71 L at 0 °C and 100 kPa, the current IUPAC standard. At 25 °C and 100 kPa it takes up 24.79 L.
FAQ
What is Avogadro’s number?
Avogadro’s number is 6.022 140 76 × 10²³, the number of particles in one mole of a substance. Since 2019 it has been exact by definition.
What is the value of R?
The molar gas constant R is 8.314 J/(mol·K), which is the same as 8.314 L·kPa/(mol·K) or 0.08206 L·atm/(mol·K).
What is the mass of a proton?
A proton has a mass of 1.6726 × 10⁻²⁷ kg, about 1.0073 u. The neutron is slightly heavier, at 1.6749 × 10⁻²⁷ kg, and the electron is about 1,836 times lighter than the proton.