Equivalent Weight Calculator

The Mass That Delivers One Reactive Unit

Molar mass tells you the weight of one mole of a compound. Equivalent weight tells you the weight that delivers exactly one mole of reactive capacity — one mole of H+, one mole of OH−, or one mole of transferred electrons, depending on the reaction. For a compound that only reacts one way per molecule, the two numbers are identical; for anything else, equivalent weight is smaller.

The Formula

The n-factor is the same value used in normality calculations — the number of reactive equivalents each formula unit provides.

Where This Calculation Matters

• Preparing a normal solution by weight — weighing out one equivalent weight of a substance and dissolving it to one liter produces a 1 N solution directly.
• Comparing acid or base strength gram-for-gram — equivalent weight normalizes different compounds to their actual reactive capacity per gram.
• Redox titration calculations — equivalent weight for a redox species depends on the number of electrons transferred, which can differ from its acid-base equivalent weight.
• Water hardness and alkalinity reporting — hardness is traditionally reported using the equivalent weight of calcium carbonate as a reference standard.

Equivalent Weights of Common Acids and Bases

Phosphoric acid's equivalent weight is a third of its molar mass because it can donate up to three protons per molecule — though in practice, only the protons that actually react in a given reaction should be counted toward n.

How to Use This Calculator

1. Enter the compound's Molar Mass (g/mol).
2. Enter the n-factor (valence / equivalents per mole) relevant to the reaction in question.
3. Select Calculate to get the equivalent weight in grams per equivalent.

Related Calculations

Need the molar mass first? Use the Molar Mass Calculator. To convert an equivalent weight-based preparation into normality, see the Normality Calculator.