Equivalent weight

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Equivalent weight is the atomic weight of an element or radical divided by the valence it assumes in a chemical compound. When choosing primary standards in analytical chemistry, compounds with higher equivalent weights are generally more desirable because weighing errors are reduced or minimized.

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 60.0092 and assumes valence 2 in compounds, so its equivalent weight is 30.0046.

The equivalent weight of a substance may be defined as that weight of it which will react with or produce one mole of hydrogen. (Or 1g of H since 1mol H=1g H)

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