The gram equivalent concept runs parallel to the mole concept, but is built on equivalent mass rather than molecular mass. The gram equivalent mass of an element, compound or ion is defined as the mass of it that combines with, or displaces, 1.008 g of hydrogen, 8 g of oxygen, or 35.5 g of chlorine.
Since referencing every reaction back to H/O/Cl directly is impractical, the general working formula is:
Gram equivalent mass = (Molar mass, g mol⁻¹) ÷ (Equivalence factor n, eq mol⁻¹)
The equivalence factor n takes a different specific meaning for each type of chemical entity:
- Acid: n = basicity = number of moles of ionisable H⁺ per mole of acid (H₂SO₄: n = 2, gram equivalent mass = 98/2 = 49 g eq⁻¹).
- Base: n = acidity = number of moles of ionisable OH⁻ per mole of base (KOH: n = 1, gram equivalent mass = 56/1 = 56 g eq⁻¹).
- Oxidising/reducing agent: n = number of moles of electrons gained/lost per mole of reagent in the redox half-reaction (KMnO₄ in acid medium, MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O: n = 5, gram equivalent mass = 158/5 = 31.6 g eq⁻¹).
Equivalent mass has no unit; gram equivalent mass has the unit g eq⁻¹.
Why use this alongside the mole concept? The mole concept needs a fully balanced equation before you can relate reactant amounts. The gram equivalent concept does not -- by definition, equivalent amounts of any two reacting substances always react in a 1:1 ratio, whatever the underlying reaction. This makes it especially convenient for redox reactions (where balancing the full equation is laborious), while the mole concept remains the natural choice for non-redox reactions.
Gram equivalent mass = molar mass ÷ equivalence factor (basicity for acids, acidity for bases, electrons transferred for redox agents); equivalents always react 1:1, without needing a balanced equation.