We are asked to calculate for the **percent by mass (mass %) of Fe ^{3}^{+} ion** in a sample of iron (II) oxide which is 75.01% iron by mass .

We’re going to use the **mass percent formula** shown below:

$\overline{){\mathbf{mass}}{\mathbf{}}{\mathbf{percent}}{\mathbf{}}{\mathbf{=}}\frac{\mathbf{mass}\mathbf{}\mathbf{component}}{\mathbf{total}\mathbf{}\mathbf{mass}}{\mathbf{\times}}{\mathbf{100}}}$

Despite Dalton’s laws, it is now known that many ionic compounds do not always contain atoms in small integer
ratios. For example, a sample of iron(II) oxide may, in fact, contain a significant amount of Fe^{3+} in addition to Fe^{2+}. This is an example of a *nonstoichiometric* compound. A sample of iron(II) oxide is found to be 75.01 % iron by mass.

Determine the percentage of Fe^{3+} ions in the sample.

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