Page 39 - Modern Analytical Chemistry

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22 Modern Analytical Chemistry

2C.1 Conservation of Mass
The easiest principle to appreciate is conservation of mass. Except for nuclear reac-
tions, an element’s total mass at the end of a reaction must be the same as that pres-
ent at the beginning of the reaction; thus, an element serves as the most fundamen-
tal reaction unit. Consider, for example, the combustion of butane to produce CO 2
and H 2 O, for which the unbalanced reaction is
C 4 H 10 (g) +O 2 (g) ® CO 2 (g) +H 2 O(g)
All the carbon in CO 2 comes from the butane, thus we can select carbon as a reac-
tion unit. Since there are four carbon atoms in butane, and one carbon atom in
CO 2 , we write

4 ´moles C 4 H 10 =1 ´moles CO 2
Hydrogen also can be selected as a reaction unit since all the hydrogen in butane
ends up in the H 2 O produced during combustion. Thus, we can write

10 ´moles C 4 H 10 =2 ´moles H 2 O
Although the mass of oxygen is conserved during the reaction, we cannot apply
equation 2.3 because the O 2 used during combustion does not end up in a single
product.
Conservation of mass also can, with care, be applied to groups of atoms. For
+
example, the ammonium ion, NH 4 , can be precipitated as Fe(NH 4 ) 2 (SO 4 ) 2 × 6H 2 O.
Selecting NH 4 as the reaction unit gives
+
+
2 ´moles Fe(NH 4 ) 2 (SO 4 ) 2 · 6H 2 O =1 ´moles NH 4

2C.2 Conservation of Charge
The stoichiometry between two reactants in a precipitation reaction is governed by
a conservation of charge, requiring that the total cation charge and the total anion
charge in the precipitate be equal. The reaction units in a precipitation reaction,
therefore, are the absolute values of the charges on the cation and anion that make
up the precipitate. Applying equation 2.3 to a precipitate of Ca 3 (PO 4 ) 2 formed from
3–
2+
the reaction of Ca and PO 4 , we write
2+ 3–
2 ´moles Ca =3 ´moles PO 4

2C. 3 Conservation of Protons
In an acid–base reaction, the reaction unit is the proton. For an acid, the num-
ber of reaction units is given by the number of protons that can be donated
to the base; and for a base, the number of reaction units is the number of
protons that the base can accept from the acid. In the reaction between H 3 PO 4
and NaOH, for example, the weak acid H 3 PO 4 can donate all three of its pro-
tons to NaOH, whereas the strong base NaOH can accept one proton. Thus,
we write
3 ´moles H 3 PO 4 =1 ´moles NaOH

Care must be exercised in determining the number of reaction units associ-
ated with the acid and base. The number of reaction units for an acid, for in-
stance, depends not on how many acidic protons are present, but on how many