In chemistry an equivalent is the amount of one substance that reacts with one mole of another chemical. Equivalent weights in chemistry are based on the atomic weight of substances, thereby making equivalents have different weights for the same amount of atoms.
For example, an element must weigh approximately 1 gram to replace an equivalent weight in one mole of hydrogen, whereas the same element must weigh approximately 8 grams to displace an equivalent weight in one mole of oxygen. The difference is that oxygen is heavier than hydrogen. Equivalent weights are commonly used in stochiometry, the study of the relationship between quantities of reactants and products in a chemical reaction.