The formula fe_2o_3 corresponds to iron oxide with the composition Fe2O3, containing 2 iron atoms and 3 oxygen atoms per formula unit.
Chemical identity
Fe2O3 is a binary ionic compound formed from iron cations and oxide anions (O2−). In many contexts it is called iron(III) oxide or ferric oxide.
Oxidation states and name
Oxygen in metal oxides is typically assigned an oxidation number of \(-2\). With three oxygen atoms, the total oxygen contribution is \(-6\). Electrical neutrality requires the two iron atoms to contribute \(+6\) in total, giving an average oxidation number of \(+3\) for each iron atom.
\[ 3(-2) = -6,\qquad 2x + (-6)=0 \Rightarrow 2x=6 \Rightarrow x=+3 \]
The Stock-system name therefore is iron(III) oxide. The Roman numeral indicates the \(+3\) oxidation state of iron.
Molar mass of Fe2O3
Standard atomic masses (g/mol) are used for a typical general chemistry calculation.
| Element | Atoms in Fe2O3 | Atomic mass (g/mol) | Mass contribution (g/mol) |
|---|---|---|---|
| Fe | 2 | 55.845 | \(2 \times 55.845 = 111.690\) |
| O | 3 | 15.999 | \(3 \times 15.999 = 47.997\) |
\[ M(\mathrm{Fe_2O_3}) = 2(55.845) + 3(15.999) = 111.690 + 47.997 = 159.687\ \text{g/mol} \approx 159.69\ \text{g/mol} \]
Percent composition by mass
Mass percent follows from each element’s mass contribution divided by the total molar mass.
\[ \%\,\mathrm{Fe} = \frac{111.690}{159.687}\times 100 = 69.94\% \qquad \%\,\mathrm{O} = \frac{47.997}{159.687}\times 100 = 30.06\% \]
Common pitfalls
- Confusing subscripts with oxidation numbers; the subscripts in Fe2O3 encode composition, while the \(+3\) oxidation state follows from charge balance with O2−.
- Mixing atomic masses from different tables; small differences in atomic weights shift the final percent composition by a few hundredths of a percent.
- Rounding too early; carrying extra digits until the final rounding preserves consistent mass percentages.