In science, the equivalent weight of a solution is the molecular weight of the solute, or dissolved substance, in grams divided by the valence of the solute. Equivalent weight predicts the mass of a substance that will react with one atom of hydrogen in an acid-base analysis such as a titration. You can calculate it easily, as long as you know the molecular weights of the compounds involved in the reaction.
Find the molecular weight of a compound used in a chemical reaction by looking up on the periodic table the molecular weight of each element and multiplying it by the number of the element in the compound before adding all molecular weights together. For example, the molecular weight of sodium chloride, NaCl, is 22.990 + 35.453, or 58.443.
Determine the valence of the compound. The valence refers to how many hydrogen atoms could bond with the compound. It is determined by the relationship between elements in a compound. For NaCl, the valence is 1 because only one hydrogen atom could bond with NaCl. For H2SO4, or sulfuric acid, the valence is 2 because two hydrogen atoms bond with sulfate, or SO4.
Divide the molecular weight by the valence to calculate the equivalent weight. The equivalent weight of NaCl is 58.443/1 or 58.443