The composition of a sample of wustite is Fe0.92O. What percentage of the iron is present in the form of Fe(III)?

From the valency of Fe(II) and Fe(III) it is clear that three Fe(II) ions will be replaced by two Fe(III) causing a loss of one iron. Total loss of iron from one molecule of FeO = 1 - 0.93 = 0.07. Hence, total Fe(III) present in one molecule of FeO = 2 × 0.07 = 0.14. Total number of Fe(II) and Fe(III) present in one molecule of FeO = 0.93. Hence, percentage of Fe(III) = 0.14 × 100 / 0.93 = 15.05