equivalent weight
/ ee-KWIV-uh-lent weight /
Think of a reagent's reactive 'hands'. The equivalent weight is the mass of substance that carries exactly one reactive hand — one proton to give or take, or one electron to gain or lose.
Numerically, the equivalent weight equals the molar mass divided by the number of reactive units per molecule. Sulfuric acid, with two donatable protons, has an equivalent weight of half its molar mass; a metal that loses three electrons has an equivalent weight of a third of its molar mass.
Equivalent weight is the companion concept to normality and to the older idea that 'equal numbers of equivalents react'. Its catch is the same: the value changes with the reaction, so the equivalent weight of a substance is not a fixed property the way molar mass is.
Sulfuric acid has a molar mass of 98 g/mol; because each molecule can release two protons, its equivalent weight in an acid-base reaction is 98 ÷ 2 = 49 g per equivalent.
Equivalent weight = molar mass ÷ reactive units per molecule.
A substance has only one molar mass but can have several equivalent weights, one for each reaction it might undergo. Never quote an equivalent weight without naming the reaction context.