Molar mass of carbon-12
Physico-chemicalMeasured
= 12.000 000 0126(37) × 10−3 kg mol−1
- Value
- 12.000 000 0126 × 10−3 kg mol−1
- Standard uncertainty
- 3.7 × 10−12 kg mol−1
- Relative uncertainty
- 3.1 × 10−10 (308 parts per trillion)
- Status
- Measured since 2019 (exact before)
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# molar mass of carbon-12, CODATA 2022 (NIST)
molar_mass_of_carbon_12 = 1.20000000126e-2 # kg mol^-1, standard uncertainty 3.7e-12
# SciPy: scipy.constants.physical_constants['molar mass of carbon-12']What it is
The mass of one mole of carbon-12 atoms. It was exactly 12 g/mol by definition until 2019; with the Avogadro constant fixed instead, it is now measured and differs from 12 g/mol by 1.05 parts per billion.
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.
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 | 12.000 000 0126(37) × 10−3 | 3.1 × 10−10 | +4.7 σ of 2018 |
| 2018 | 11.999 999 9958(36) × 10−3 | 3.0 × 10−10 | |
| 2014 | 12 × 10−3 | exact | exact |
| 2010 | 12 × 10−3 | exact | exact |
| 2006 | 12 × 10−3 | exact | exact |
| 2002 | 12 × 10−3 | exact | exact |
| 1998 | 12 × 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).