55.8 X 2 = 111.6
32.1 X 3 =
16.0 X 12 = + 192.0
Multiply the GMW, expressed in grams per mole, times the 1/TPIV, expressed
in moles per equivalent, used as an appropriate factor.
---------- X -------- = 66.65 g/Eq
EXCEPTIONS IN CALCULATING GRAM EQUIVALENT WEIGHT
The only exact method of calculating the GEW is to examine the complete
reaction that the molecule will undergo. For example:
H3PO4 + NaOH <=======> NaH2PO4 + HOH
In this reaction, only one of the hydrogen ions on the phosphoric acid molecule
reacted with the sodium hydroxide. The TPIV for the acid in this example is one.
H3PO4 + 2NaOH <=======> Na2HPO4 + 2HOH
In this example, two moles of sodium hydroxide were reacted with one mole of
phosphoric acid and the TPIV is two since two of the hydrogen ions from the acid
H3PO4 + 3NaOH <=======> Na3PO4 + 3HOH
The TPIV in this reaction is three since three hydrogen ions from the acid reacted
with the three moles of sodium hydroxide. A general rule to follow is to examine the
reaction that is to occur. If the products of the reaction cannot be determined by
inspecting the given material, then assume complete ionization unless other information
is given. In the case of phosphoric acid, if no guidance is given as to the type of
reaction that the molecule will undergo, or if the degree of ionization is not given, then
assume the TPIV to be three.
An equivalent indicates the number of grams equal to the equivalent weight of
the substance. One equivalent has the mass of one gram equivalent weight. The
mechanism of solving for the number of equivalents is similar to that of solving for the
number of moles except GEW is used instead of GMW.