Equivalent Weight
Definition and meaning of Equivalent Weight in chemistry.
Equivalent weight is the mass of a substance that reacts with or supplies one mole of a specified reactive unit, one mole of H+ ions in acid-base chemistry, or one mole of electrons in a redox reaction.
In more detail
It is calculated by dividing a substance's molar mass by its n-factor (equivalence factor), the number of reactive units each formula unit provides. For acids and bases, the n-factor equals the number of replaceable protons or hydroxide ions; for oxidizing or reducing agents, it equals the number of electrons transferred. Because the n-factor depends on the reaction, one substance can have different equivalent weights in different contexts. The concept underlies normality (N), a concentration unit still used in analytical titrations.
Key facts
| Field | General Chemistry |
|---|---|
| Formula | Equivalent weight = Molar mass ÷ n-factor |
| Unit | grams per equivalent (g/eq) |
| Related quantity | Normality (N) = Molarity × n-factor |
Sulfuric acid (H2SO4) has a molar mass of 98.08 g/mol. Since it donates two H+ ions per molecule in complete neutralization (n = 2), its equivalent weight is 98.08 / 2 = 49.04 g/equivalent.
Frequently asked questions
How does equivalent weight relate to normality?
Normality equals molarity multiplied by the n-factor, so equivalent weight is the mass corresponding to one equivalent, just as molar mass corresponds to one mole.
Why can one compound have more than one equivalent weight?
Because the n-factor depends on the specific reaction, phosphoric acid (H3PO4), for instance, has an n-factor of 1, 2, or 3 depending on how many of its protons are neutralized.