dalton (as molar mass)
The dalton, used as a molar mass, is 1.00000000105 × 10⁻³ kg/mol. Strictly the dalton is a unit of MASS, one twelfth of a carbon-12 atom, but a species of molecular mass m daltons has a molar mass of numerically m grams per mole, and biochemistry uses the two interchangeably.
Watch out: That leftover is one part in a billion and matters to nobody outside metrology, but it is the reason this entry is not exact. What does matter is remembering that Da is a mass: a 50 kDa protein has a mass of 8.3 × 10⁻²⁰ g, and only its molar mass is 50 kg/mol.
| 1 Da | 1 g/mol |
The equivalence used to be exact by definition: before 2019 the mole was defined as the number of atoms in 12 g of carbon-12, which forced the molar mass constant to be exactly 1 g/mol. The 2019 redefinition fixed the Avogadro constant instead, so the constant became a measured quantity of 1.00000000105 g/mol.