Q.The equivalent mass of a trivalent metal element is 9 g eq⁻¹. The molar mass of its anhydrous oxide is

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).