On 20 May 2019, the SI definition of mole changed in such a way that the molar mass constant remains nearly but no longer exactly 1 g/mol.
(The speed of light is another example of a physical constant whose value is fixed by the definitions of the International System of Units (SI).
One consequence of this change is that the previously defined relationship between the mass of the 12C atom, the dalton, the kilogram, and the Avogadro number is no longer exact.
One of the following had to change: The wording of the 9th SI Brochure[Note 1] implies that the first statement remains valid, which means the second is no longer exactly true.
Appendix 2 to the 9th SI Brochure states that "the molar mass of carbon 12, M(12C), is equal to 0.012 kg⋅mol−1 within a relative standard uncertainty equal to that of the recommended value of NAh at the time this Resolution was adopted, namely 4.5×10−10, and that in the future its value will be determined experimentally",[8][9] which makes no reference to the dalton and is consistent with either statement.