Section I. EQUIVALENT WEIGHT
a. A gram equivalent weight by definition is the mass of substance that will
combine with or displace one mole of hydrogen.
b. In solutions of ionic compounds, the compound dissociates into positive or
negative ions to react with other ions. An example is KOH, which separates into one K+
ion and one OH- ion. One mole of K+ will replace one mole of H+ in a chemical reaction;
hence, one equivalent weight of KOH is equal to one mole of KOH. Consider now the
combining ability of the SO4-2 ion in sulfuric acid, H2SO4. One mole of the SO42 ion will
combine with two moles of H+. Since one gram equivalent weight of a compound or
element is the mass that will combine with one mole of H+, then one gram equivalent
weight of H2SO44 equals 0.5 moles (1/2 the gram molecular weight) of H2SO4 because
two H ions will combine with one SO4-2 ion. Total positive ionic valence (TPIV)
expresses the number of equivalents per mole of substance.
NOTE: This is true for metathesis reactions only, where ionic valence does not change.
DETERMINING THE GRAM EQUIVALENT WEIGHT
In solving for gram equivalent weight (GEW), the first step is to find the GMW of the
compound and then divide by the total positive ionic valence (TPIV). The TPIV is normally
determined by finding the valence of the first ion in the chemical compound. (See Appendix C
for the valences of common ions.) The valence of that ion is then multiplied by any subscripts
pertaining to that ion. If two or more valences are possible for the ion, simply find the
valence of the negative ion. Multiply the valence by any subscripts that pertain to that
ion giving a total negative ionic valence. Since the sum of the positive valence and
negative valence is zero, the TPIV is numerically equal to the total negative valence.
a. Example 1. Find the GEW of KOH.
Solution. Determine the GMW of the compound