Question

The composition of a sample of wustite is $Fe _{0.93} O$. What percentage of the iron is present in the form of Fe (III)?

• A. $5\%$
• B. $7.08\%$
• C. $15.05\%$
• D. $23.6\%$

Answer 1

Answer: (C)

$15.05\%$

Answer 2

In pure iron oxide $(FeO)$, iron and oxygen are present in the ratio $1: 1$. However, here number of $Fe ^{2+}$ present $=0.93$ Or No. of $Fe ^{2+}$ ions missing $=0.07$ Since each $Fe ^{2+}$ ion has $2$ positive charge, the total number of charge due to missing $( 0.07 ) Fe ^{2+}$ ions $=0.07 \times 2=0.14$ To maintain electrical neutrality, $0.14$ positive charge is compensated by the presence of $Fe ^{3+}$ ions. Now since, replacement of one $Fe ^{2+}$ ion by one $Fe ^{3+}$ ion increases one positive charge, $0.14$ positive charge must be compensated by the presence of $0.14 \,Fe ^{3+}$ ions. In short, $0.93\, Fe ^{2+}$ ions have $0.14\, Fe ^{3+}$ ions $100 \,Fe ^{2+}$ ions have $=\frac{0.14}{0.93} \times 100=15.05 \%$