Use the atomic weight in the periodic table to determine the equivalent weight of magnesium.

The atomic weight of Magnesium (Mg) is about 24.305 g / mol.

Magnesium when forms an ion can donate two electrons hence forming Mg2+. Since it can donate two electrons, therefore it has an equivalence of 2 equivalence per mole, therefore:

equivalent weight = (24.305 g / mol) * (1 mol / 2 equivalence)

equivalent weight = 12.1525 g / equivalence