Equivalent and the law of equivalents

Substances react with one another in equal-valued — equivalent — amounts. The equivalent mass of an element (the book says «equivalent weight») is the mass of the element that combines with 1 part by mass of hydrogen or 8 parts by mass of oxygen: E = A/V, where A is the atomic mass and V the valence; hence E(H) = 1, E(O) = 8, E(Cl) = 35.5. If one element shows different valences in different compounds, its equivalent mass differs too (sulfur: 32/4 = 8 in SO₂ and 32/6 ≈ 5.33 in SO₃). For compounds: oxide E = M/(n·V) (n is the element’s subscript, V its valence); acid E = M/n(H) (the number of H replaceable by a metal); base E = M/n(OH); salt E = M/(n·V) (by the metal); ion E = M/z. Law of equivalents: the masses of reacting substances are proportional to their equivalent masses, m₁/m₂ = E₁/E₂. For gases an equivalent volume is also used: under normal conditions it is 11.2 L for hydrogen and 5.6 L for oxygen, since 1 g of H₂ and 8 g of O₂ occupy these volumes. Equivalent mass is expressed in g/mol (the book writes g/equiv). Note: «equivalent» and «equivalent weight» are older terms; current international (IUPAC) practice uses the mole and the reaction equation instead, but they still appear in school and test problems.