Determination of Reaction Stoichiometry:
An Application of the Method of Continuous Variation
Various volumes of 10 mM iron(III) nitrate and 10 mM
sodium salicylate are mixed in separate test tubes in ratios shown
in Table 1 (found in the online material). The total volume of
these two solutions is a constant 1.00 mL, keeping the total
amount of reagents the same. Dilute nitric acid (4.00 mL, ~60
mM) is added to each mixture, and the absorbance at λmax is
recorded for each solution. A typical plot of absorbance versus
the mole fraction of iron(III) is shown in Figure 2. An apparent
local maximum around 0.250 mole fraction (a 1:3 iron:salicylate
complex) is a typical result, which may represent a minor complex,
but the overall maximum in this plot at 0.500 mole fraction
(a 1:1 iron:salicylate complex) indicates the predominant
stoichiometry of the reaction.