Page 38 - Modern Analytical Chemistry

1400-CH02 9/8/99 3:48 PM Page 21

Chapter 2 Basic Tools of Analytical Chemistry 21

EXAMPLE 2.6
The amount of oxalic acid in a sample of rhubarb was determined by reacting
with Fe 3+ as outlined in reaction 2.2. In a typical analysis, the oxalic acid in
10.62 g of rhubarb was extracted with a suitable solvent. The complete
oxidation of the oxalic acid to CO 2 required 36.44 mL of 0.0130 M Fe . What
3+
is the weight percent of oxalic acid in the sample of rhubarb?
SOLUTION
We begin by calculating the moles of Fe used in the reaction
3+
0 0130 mol Fe 3+ –4 3+
.
.
´ 0 03644 L = 4.737 ´ 10 mol Fe
L
3+
The moles of oxalic acid reacting with the Fe , therefore, is
2
–4
–4
4.737 ´ 10 mol Fe 3+ ´ 1 mol C 2 HO 4 = 2.369 ´ 10 mol C 2 HO 4
2
2 mol Fe 3+
Converting moles of oxalic acid to grams of oxalic acid
90.03 g C 2 HO 4 –2
2
–4
g
2.369 ´ 10 mol C 2 HO 4 ´ = 2.132 ´ 10 oxalic acid
2
mol C 2 HO 4
2
and converting to weight percent gives the concentration of oxalic acid in the
sample of rhubarb as
–2
2.132 ´ 10 g C 2 HO 4
2
´100 =0 201% ww C 2 H O 4
.
/
2
g
.
10 62 rhubarb
In the analysis described in Example 2.6 oxalic acid already was present in the
desired form. In many analytical methods the compound to be determined must be
converted to another form prior to analysis. For example, one method for the quan-
titative analysis of tetraethylthiuram disulfide (C 10 H 20 N 2 S 4 ), the active ingredient in
the drug Antabuse (disulfiram), requires oxidizing the S to SO 2 , bubbling the SO 2
through H 2 O 2 to produce H 2 SO 4 , followed by an acid–base titration of the H 2 SO 4
with NaOH. Although we can write and balance chemical reactions for each of these
steps, it often is easier to apply the principle of the conservation of reaction units.
A reaction unit is that part of a chemical species involved in a reaction. Con-
sider, for example, the general unbalanced chemical reaction
A +B ® Products
Conservation of reaction units requires that the number of reaction units associated
with the reactant A equal the number of reaction units associated with the reactant
B. Translating the previous statement into mathematical form gives
Number of reaction units per A ´moles A 2.3
=number of reaction units per B ´moles B

If we know the moles of A and the number of reaction units associated with A and
B, then we can calculate the moles of B. Note that a conservation of reaction units,
as defined by equation 2.3, can only be applied between two species. There are five
important principles involving a conservation of reaction units: mass, charge, pro-
tons, electron pairs, and electrons.