Iron oxide has formula Fe0.94O.
Let there be 100 oxide ions and 94 iron ions.
Fe+x:O−2=94:100
Total negative charge on 100 oxide ions =−200
It means as per the concept of charge neutrality, 94 iron ions should have total charge of +200.
Let number of Fe+3=X
∴ Number of Fe+2=(94−X)
Total positive charge =3X+2(94−X)=200⇒X=12
∴ Percentage of Fe+3=1294×100=12.8%