Molecular (or formula) mass from atomic masses (Mr = sum of (atoms of element i × atomic mass of i))
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Mr = sum of (atoms of element i × atomic mass of i)
What each symbol means
| Symbol | What it stands for |
|---|---|
| A_i | Standard atomic weight of element i (u or g/mol numerically) |
| M_r | Relative molecular (or formula) mass (dimensionless, same numerical value as molar mass in g/mol) |
| n_i | Number of atoms of element i in one formula unit |
When to use this
For molecular compounds use molecular formula; for ionic solids use formula unit (e.g. NaCl). Isotopic abundance is taken as natural average.