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Gram-equivalent weight

Def. Gram-equivalent weight. The mass of a substance, in grams, that combines with or is chemically equivalent to, one gram-atomic weight (1.008 grams) of hydrogen: the atomic or molecular weight divided by the valence.

Syn. Equivalent weight, gram-equivalent

Gram-equivalent weights for various cases:

1. Complex ion. The gram-equivalent weight of a complex ion is equal to the formula weight of the ion divided by its valence.

Examples. The gram-equivalent weight of the OH - ion is 16.000 + 1.008 = 17.008 g and of the Ca++ ion is 40.08/2 = 20.04 g.

2. Acid. The gram-equivalent weight of an acid is that weight of the acid that contains 1 gram-equivalent weight (1.008 g) of replaceable hydrogen.

Examples. 1) The gram-equivalent weight of HCl is one mole of HCl (1.008 + 35.453 = 36.461 g). 2) The gram-equivalent weight of H2SO4 is usually ½ mole since both hydrogens are replaceable in most reactions of sulfuric acid. 3) The gram-equivalent weight of H3PO4 may be 1 mole, ½ mole, or ⅓ mole, depending on whether 1, 2, or 3 hydrogen atoms per molecule are replaced in a particular solution. 4) The gram-equivalent weight of H3BO3 is always one mole, since only one hydrogen is replaceable.

There are no simple rules for predicting how many hydrogens of an acid will be replaced in a given reaction.

3. Base. The gram-equivalent weight of a base is that weight of the base that contains 1 gram-equivalent weight (17.008 g) of replaceable OH - .

Examples. The gram-equivalent weight of NaOH is 1 mole (i.e. 1 gram-formula weight) , that of Mg(OH)2 is ½ mole, and of Al(OH)3 is ⅓ mole.

4. Simple normal salt. The gram-equivalent weight of a simple normal salt that does not act as an oxidizing or reducing agent is that weight of the salt which contains 1 gram-equivalent weight of the metal or acid radical.

Examples. The gram-equivalent weight of NaCl is 1 mole (i.e. 1 gram-formula weight), that of NH4Cl is1 mole, that of CaCl2 is ½ mole, that of AlF3 is ⅓ mole, and that of Mg3(PO4)•4H2O is 1/6 mole.

In general, the gram-equivalent weight of a salt in which a change of valence does not occur during the reaction is equal to its gram-formula weight divided by the total charge of all positive ions (or of all negative ions).

5. Acid and complex salts. The gram-equivalent weights of acid and complex salts depend on the metal, the unreplaced acid hydrogen, or the acid radical, with reference to which the equivalent is to be calculated.

Example. The gram-equivalent weight of the acid salt NaH2PO4 would be 1 mole (i.e. 1 gram-formula weight), ½ mole, or ⅓ mole, depending on whether the salt is used for its Na, total H or PO4 content.

Equivalent weight of an oxidizing or reducing agent. The equivalent weight of an oxidizing or reducing agent for a particular reaction is equal to its formula weight divided by the total number of electrons gained or lost when the reaction occurs (i.e. by the total change in valence).

Example. Q. In a given reaction MnO2, in HCl, solution, is reduced to MnCl2. What is the equivalent weight of MnO2 in this reaction?

A. In the reaction

            MnO2 + 2 HCl →MnCl2 + H2O

Mn has a valence of +4 in MnO2 and a valence of +2 in MnCl2. The change in its valence is thus 2 and


Note that Mn is the only element in MnO2 whose valence changes during the reaction (by convention the valence of oxygen is always -2 except in peroxides). Thus the change in valence of Mn represents the total change in valence of the atoms comprising MnO2.

A given oxidizing or reducing agent may have more than one equivalent weight, depending on the reaction for which it is used.


 Schaum, Beckman, Rosenberg. College Chemistry. (Schaum)

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