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Dear student,.... In pure iron oxide (FeO), iron and oxygen are present in the ratio 1 : 1. However, here number of Fe2+ present = 0.93 Or No. of Fe2+ ions missing = 0.07 Since each Fe2+ ion has 2 positive charge, the total number of charge due to missing (0.07) Fe2+ ions = 0.07 * 2 = 0.14 To maintain electrical neutrality, 0.14 positive charge is compensated by the presence of Fe3+ ions. Now since, replacement of one Fe2+ ion by one Fe3+ ion increases one positive charge, 0.14 positive charge must be compensated by the presence of 0.14 Fe3+ ions. In short, 0.93 Fe2+ ions have 0.14 Fe3+ ions 100 Fe2+ ions have = 0.14/0.93 * 100 = 15.05%.....Regards.