Molar mass constant
Physico-chemicalMeasured
= 1.000 000 001 05(31) × 10−3 kg mol−1
- Value
- 1.000 000 001 05 × 10−3 kg mol−1
- Standard uncertainty
- 3.1 × 10−13 kg mol−1
- Relative uncertainty
- 3.1 × 10−10 (310 parts per trillion)
- Status
- Measured since 2019 (exact before)
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# molar mass constant, CODATA 2022 (NIST)
molar_mass_constant = 1.00000000105e-3 # kg mol^-1, standard uncertainty 3.1e-13
# SciPy: scipy.constants.physical_constants['molar mass constant']What it is
Before 2019 this was exactly 1 g/mol. Now that the Avogadro constant is fixed instead, M_u = N_A·m_u is measured, and it differs from 1 g/mol by 1.05 parts per billion, which matters only for the most exacting chemistry.
Until CODATA 2014 this value was exact by definition. Since the 2019 SI redefinition it is measured, because the constant that used to fix it was replaced by a fixed elementary charge.
Equations
- definition
How the value changed, 1998–2022
Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart).
Relative uncertainty
| CODATA | Value (uncertainty) | Relative uncertainty | Change from previous |
|---|---|---|---|
| 2022 | 1.000 000 001 05(31) × 10−3 | 3.1 × 10−10 | +4.7 σ of 2018 |
| 2018 | 0.999 999 999 65(30) × 10−3 | 3.0 × 10−10 | |
| 2014 | 1 × 10−3 | exact | exact |
| 2010 | 1 × 10−3 | exact | exact |
| 2006 | 1 × 10−3 | exact | exact |
| 2002 | 1 × 10−3 | exact | exact |
| 1998 | 1 × 10−3 | exact |
“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).