According to the UCLA Department of Chemistry and Biochemistry, the equivalent weight is the weight required to provide the equivalent of one proton or one hydroxide anion. You can calculate this in a few minutes using a formula and some information about the molecule in question. You need the molecular formula and a copy of the periodic table for this calculation. The following method shows how to calculate the theoretical equivalent weight.
Find the atomic weights for each element in the molecule
Locate this number under the atomic symbol on the periodic table. For example, carbon, with the atomic symbol C, has an atomic weight of 12.011 grams per mole.
Calculate the molecular weight of the substance
Multiply the atomic weight by the element's subscript in the molecular formula and add. In hydrates, multiply the weight of the water molecule by the number located between the dot and the H2O and add to the total.
Calculate equivalent weight
Divide the molecular weight by the number of hydrogen protons it donates as acid or the number of hydroxide (OH)- anions it produces as a base. Use the units of grams per equivalent of H+ or grams per equivalent of (OH)-, depending on the ion species the molecule donates to the reaction.