- The mass of a chemical element or compound which, in a reaction, combines with or displaces one mole of hydrogen. For instance, one mole of chlorine combines with one mole of hydrogen to form hydrochloric acid. In this example, the equivalent weight of chlorine is approximately 35.45 grams, as that is the weight of one mole of chlorine. In the case of an element, the equivalent weight is equal to its gram-atomic weight divided by its valence. In the case of a molecule, it is the gram-molecular weight divided by the effective valence. Also called gram-equivalent.