Molar Masses of Elements
The molar mass of atoms of an element is given by the atomic weight of the element multiplied by the molar mass constant, M
u = 1×10−3 kg/mol = 1 g/mol:
- M(H) = 1.007 97(7) × 1 g/mol = 1.007 97(7) g/mol
- M(S) = 32.065(5) × 1 g/mol = 32.065(5) g/mol
- M(Cl) = 35.453(2) × 1 g/mol = 35.453(2) g/mol
- M(Fe) = 55.845(2) × 1 g/mol = 55.845(2) g/mol.
Multiplying by the molar mass constant ensures that the calculation is dimensionally correct: atomic weights are dimensionless quantities(i.e., pure numbers) whereas molar masses have units (in this case, grams/mole).
Some elements are usually encountered as molecules, e.g. hydrogen (H
2), sulfur (S
8), chlorine (Cl
2). The molar mass of molecules of these elements is the molar mass of the atoms multiplied by the number of atoms in each molecule:
- M(H
2) = 2 × 1.007 97(7) × 1 g/mol = 2.015 88(14) g/mol - M(S
8) = 8 × 32.065(5) × 1 g/mol = 256.52(4) g/mol - M(Cl
2) = 2 × 35.453(2) × 1 g/mol = 70.906(4) g/mol.
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