Gram Equivalent

GRAM EQUIVALENT of a given substance is the amount of this substance expressed in grams that is equivalent to one gram-atom (i.e., 1.008 g) of hydrogen. For example, in the compound of chlorine with hydrogen (HCl), the gram-equivalent of chlorine is 35.46 g: 35.46 g of chlorine are combined with 1.008 g of hydrogen. Thus, the gram-equivalent of a monovalent element is numerically equal to its atomic weight, of a divalent element is half the atomic weight, of a trivalent element is one-third, and so on. For an element combined not with hydrogen but with some other element, the equivalent is determined indirectly: thus, for carbon in carbon dioxide (CO₂), the gram-equivalent is determined as the amount of carbon equivalent to the gram-equivalent of oxygen, i.e., 8 g of oxygen, and is therefore equal to ¹²/₄=3 g (12 is the atomic weight of carbon). It is self-evident that the gram-equivalent of an element exhibiting variable valence in different compounds is not the same: for example, in carbon monoxide (CO), the gram-equivalent of carbon is not 3, but 6 g. The concept of gram-equivalent is extremely useful and convenient when applied to complex molecules as a whole. For monobasic acids, bases, and their salts (for example HCl, NaOH, NaCl), the gram-equivalent is equal to the molecular weight; for dibasic [for example H₂SO₄, Ca(OH)₂, CaSO₄, Na₂SO₄]-half the molecular weight, and so on.