Equivalent weight is calculated as

• A. Molecular weight / Valence
• B. Valence / Molecular weight
• C. Molecular weight * Valence
• D. 1 / (Molecular weight * Valence)

Answer: (A)

Equivalent weight tells you how much of a substance reacts as one reactive unit, such as one mole of H+, one mole of electrons, or one mole of charge transfer, per mole of substance. The number of reactive units per mole is called the valence. So, to get the amount that corresponds to one equivalent, you divide the molar mass (molecular weight) by the valence. In other words, EW = molar mass / valence.

This is why the correct approach is to take the molecular weight and divide it by how many equivalents a mole of the substance can provide. For example, an acid that donates two protons per molecule has valence 2, so its equivalent weight is molar mass divided by 2. A substance that donates one unit has valence 1, so its EW equals its molar mass.