Gram Equivalent Concept

Alongside the mole concept, chemistry -- especially analytical chemistry -- makes heavy use of a second, related counting idea: the gram equivalent concept. Where the mole concept is built on molecular mass, the gram equivalent concept is built on equivalent mass.

Definition. 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, or 8 g of oxygen, or 35.5 g of chlorine.

Worked example -- zinc (Example 1.1): In the reaction Zn + H₂SO₄ → ZnSO₄ + H₂, one mole of zinc (65.38 g) displaces one mole of hydrogen molecules (2.016 g). Scaling this down proportionally to find the mass of zinc that would displace exactly 1.008 g of hydrogen:

mass of Zn = 65.38 × (1.008 ÷ 2.016) = 65.38 ÷ 2 = 32.69

So the equivalent mass of zinc is 32.69 (a pure ratio, no unit), and its gram equivalent mass is 32.69 g eq⁻¹. This distinction matters: equivalent mass itself has no unit, but gram equivalent mass is expressed in g eq⁻¹.

A more general formula. In practice it is rarely convenient to reference every reaction back to hydrogen, oxygen or chlorine directly, so chemists use an equivalent, more general expression: …