Mass percentage of an element
How much of an ore is iron?
1. Iron ores and extraction of iron
2. Which oxide is richer in iron?
Magnetite (Fe3O4) and hematite (Fe2O3) are both iron oxides. Which pure oxide has the larger share of iron?
Pick the right answer
Predict: which has the higher percentage of iron by mass?
- A. Fe3O4
- B. Fe2O3
- C. Equal: both are just iron and oxygen
الشرح
Fe3O4 has 3 Fe to 4 O (0.75 per O); Fe2O3 has 2 Fe to 3 O (about 0.67 per O). More iron per oxygen gives Fe3O4 the higher iron percentage: 72.4% against 70.0%.
3. The formula
Mass percentage of an element
Mass of the element in one mole of the compound × 100 / molar mass of the compound.
\%\,Fe = mass of Fe in 1 molmolar mass × 100
Mass of the element = number of its atoms × its atomic mass.
4. Worked example: magnetite
| Element | Fe | O | C | H |
|---|---|---|---|---|
| Atomic mass | 56 | 16 | 12 | 1 |
- Molar mass of Fe3O4 = 3 × 56 + 4 × 16 = 232 g/mol
- Mass of Fe in 1 mol = 3 × 56 = 168 g
- % Fe = 168 / 232 × 100 = 72.4%
Now try it live: change the atoms and watch the percentage.
Fe atoms 3
O atoms 4
Mass of Fe 168 g
% Fe 72.4 %
5. Your turn: siderite
Fill in the blanks
Siderite, FeCO3 (molar mass 116 g/mol): % Fe = _____ % (one decimal place).
الشرح
56 / 116 × 100 = 48.3%.
Pick the right answer
Pure Fe3O4 is 72.4% iron. The book's table gives magnetite ore 45–70% iron. Why is the ore lower?
- A. The ore holds impurities. They add mass but no iron.
- B. In the ground, the formula of Fe3O4 changes.
- C. Some iron atoms are lost when the ore is weighed.
- D. The percentage formula only works for pure metals.
الشرح
Magnetite ore is Fe3O4 mixed with impurities. They add mass but no iron, so the same iron is a smaller share of the total: 45–70% instead of 72.4%.
6. Summary
- % element = mass of the element in 1 mol × 100 / molar mass.
- Use masses, not the number of atoms.
- An ore is lower in iron than its pure compound: impurities add mass.
Back to the hook: can you explain why Fe3O4 has a larger share of iron than Fe2O3?
Reveal
Fe3O4 has 3 Fe for 4 O (0.75 per O); Fe2O3 has 2 Fe for 3 O (about 0.67 per O). More iron per oxygen gives the larger share: 72.4% against 70.0%.