Composition of sample of wurtzite is  ${\mathrm{Fe}}_{0.93}{\mathrm{O}}_{1.0}$ what percentage of iron is present in the form of Fe (III)? 

 

In pure form, FeO has Fe²⁺ and O^2- which present in 1:1 ratio. According to given formula, number of Fe²⁺ has 0.93, which means that 0.07 number of Fe²⁺ are missing. Missing charge due to 0.07 of Fe²⁺ is calculated as:

$0.07\times 2=0.14$

Equal number of Fe²⁺ ions have been replaced by equal number of Fe³⁺ ions. 

In the given formula, 0.93 Fe²⁺ ions have 0.14 Fe³⁺ ions

$100 {\mathrm{Fe}}^{2+} \mathrm{ions}=0.93 {\mathrm{Fe}}^{2+}+ 0.14 {\mathrm{Fe}}^{3+}=\frac{0.14}{0.93}\times 100=15.05\%$