Molar mass
Adding atomic masses, ready for iron ores
1. Molar mass: from zero
2. How heavy is one mole of ore?
The book's ore table lists Fe2O3, Fe3O4 and FeCO3. To compare their iron, we need the mass of one mole of each.
A mole is a fixed count of particles (Avogadro's number, 6.02 × 10²³).
Pick the right answer
Predict: Fe = 56 and O = 16. What is the mass of one mole of Fe2O3?
- A. 72 g
- B. 160 g
- C. 5 g
Explanation
One mole of Fe2O3 holds 2 Fe and 3 O: 2 × 56 + 3 × 16 = 112 + 48 = 160 g.
3. Molar mass
Molar mass
The sum of atomic masses of the elements forming the molecule or formula unit, measured in grams (g/mol).
| Element | Fe | O | C | H |
|---|---|---|---|---|
| Atomic mass | 56 | 16 | 12 | 1 |
Multiply each atomic mass by the number of its atoms. Then add.
4. Worked example: hematite
M(Fe2O3) = 2 × 56 + 3 × 16 = 112 + 48 = 160\ g/mol
Limonite has water of crystallization. The dot means: add the water's mass too.
Show limonite
M(2Fe2O3*3H2O) = 2 × 160 + 3 × 18 = 374\ g/mol
Each H2O is 2 × 1 + 16 = 18. The numbers in front multiply the whole unit.
5. Build an iron oxide
Set the number of Fe and O atoms. Try FeO, Fe3O4 and Fe2O3.
Fe atoms 1
O atoms 1
Mass of Fe 56 g
Molar mass 72 g/mol
6. Your turn: siderite and CO₂
Fill in the blanks
Siderite, FeCO3: molar mass = _____ g/mol (Fe = 56, C = 12, O = 16).
Explanation
56 + 12 + 3 × 16 = 116 g/mol.
Pick the right answer
Coke burns to CO2 in the blast furnace. What is the molar mass of CO2?
- A. 28 g/mol
- B. 30 g/mol
- C. 44 g/mol
- D. 88 g/mol
Explanation
CO2 has one C and two O: 12 + 2 × 16 = 44 g/mol.
7. Summary
- Molar mass = sum of the atomic masses in the formula, in g/mol.
- A subscript multiplies the atom, or the bracketed group, just before it.
- In a hydrate, add the mass of the water of crystallization.
Back to the hook: can you explain why one mole of Fe2O3 has a mass of 160 g?
Reveal
Fe2O3 has 2 Fe atoms and 3 O atoms: 2 × 56 + 3 × 16 = 112 + 48 = 160 g/mol, so one mole has a mass of 160 g.