Video: Applying Knowledge of Empirical Formulas of Iron Oxides and Oxidation States

For statements I and II, state for each if they are true or false. I) FeO and Fe₂O₃ have the same empirical formula. II) In FeO and Fe₂O₃ oxygen has the same oxidation state. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) FeO and Fe₂O₃ have the same empirical formula. II) In FeO and Fe₂O₃ oxygen has the same oxidation state. If both are true, state if II is a correct explanation for I.

Fe is the chemical symbol for iron and O is the symbol for oxygen. So, what we’re dealing with here are two iron oxides. The empirical formula of a compound shows the atoms or ions of each element in their simplest ratio. For instance, hydrogen peroxide has the molecular formula H₂O₂. But the empirical formula of hydrogen peroxide is only HO. The empirical formula has a simplified ratio of one hydrogen atom for every oxygen atom. The word empirical in empirical formula derives from the fact that empirical formulas are determined from experimental data. The masses of individual elements of a compound are measured and then the formula is determined from that.

Now, let’s go back to our iron oxides. We can think about the formula for FeO as telling us that we only have one of each, one iron and one oxygen. So, the ratio of iron to oxygen is one to one. And for Fe₂O₃, the ratio is two to three. These two ratios are different and they’re in their simplest form. Therefore, statement I is false. The empirical formula of FeO is FeO and the empirical formula of Fe₂O₃ is Fe₂O₃.

The next statement says that in our two iron oxides, oxygen has the same oxidation state. These compounds are examples of metal-nonmetal compounds. So, we expect them to be ionic, and the oxidation state of a simple ion is the same as its charge. So, all we need to do is figure out what charge oxygen would have. The element oxygen can be found in group 16 in the periodic table, sometimes called group six. Oxygen atoms have six valence electrons, and they readily accept two electrons to form O²⁻ ions with a full outer shell. So, we would expect in the two iron oxides for oxygen to have the same oxidation state, minus two.

Remember, for charges, we put the plus or minus on the right. And for oxidation states, we put the plus or minus on the left. This is so we know which is which. Statement II is therefore true. What is different about the oxidation states in these iron oxides is the oxidation state of the iron. In FeO, the iron has an oxidation state of plus two and a charge of two plus. So, we have iron two oxide. While in Fe₂O₃, iron has an oxidation state of plus three, so it’s iron three oxide. Since only one of the statements is true, we don’t have to address the last part of the question.

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