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by the amount of substance in exactly 12 grams of carbon-12. Effective 20 May 2019, the BIPM defined the Avogadro constant as the exact value , thus redefining the mole as the amount of a substance having exactly constituent particles of that substance. One consequence of this change is that the mass of a mole of ¹²C atoms is no longer exactly 0.012 kg. On the other hand, the dalton, Da ( unified atomic mass unit, u), remains unchanged as of the mass of ¹²C. Thus, the molar mass constant

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