Equivalent weight

Equivalent weight is the atomic weight of an element or radical divided by the valence it assumes in a chemical compound.

For example, hydrogen, with atomic weight 1.008 and valence 1, has an equivalent weight of 1.008. Oxygen assumes a valence of 2 and has an atomic weight of 15.9994, so it has an equivalent weight of 7.9997.

Elements may assume different equivalent weights in different compounds. For example, iron (atomic weight 55.845) assumes equivalent weight 27.9225 if it is valence 2 in the compound (ferrous); or 18.615 if it is valence 3 (ferric).

The equivalent weight can also be computed for other than pure elements. For example, the carbonate radical (CO₃) has a formula (atomic) weight of 76.0083 and assumes valence 2 in compounds, so its equivalent weight is 38.00415.