# Molar mass

> Adding atomic masses, ready for iron ores


## Objectives

- Calculate the molar mass of a compound from its formula and atomic masses.
- Use the subscripts and coefficients in a formula correctly, including a hydrated ore.

## 1. Molar mass: from zero




## 2. How heavy is one mole of ore?

Predict: Fe = 56 and O = 16. What is the mass of one mole of $\ce{Fe2O3}$?

- 72 g
- 5 g
- 160 g ✓

One mole of $\ce{Fe2O3}$ holds 2 Fe and 3 O: 2 × 56 + 3 × 16 = 112 + 48 = 160 g.

[ar-EG] الـ mole الواحد من الـ $\ce{Fe2O3}$ فيه 2 Fe و 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). Rounded atomic masses used in exam problems Multiply each atomic mass by the number of its atoms. Then add. A subscript multiplies the atom, or the bracketed group, just before it. In $\ce{CO3}$ the 3 counts O only; in $\ce{Fe2(SO4)3}$ it counts the whole $\ce{SO4}$ group.


## 4. Worked example: hematite

Fe₂O₃: 2 Fe atoms, 3 O atoms Limonite has water of crystallization. The dot means: add the water's mass too. Show limonite Each $\ce{H2O}$ 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, $\ce{Fe3O4}$ and $\ce{Fe2O3}$. Fe atoms O atoms Mass of Fe Molar mass


## 6. Your turn: siderite and CO₂

Coke burns to $\ce{CO2}$ in the blast furnace. What is the molar mass of $\ce{CO2}$?

- 44 g/mol ✓
- 88 g/mol
- 28 g/mol
- 30 g/mol

$\ce{CO2}$ has one C and two O: 12 + 2 × 16 = 44 g/mol.

[ar-EG] الـ $\ce{CO2}$ فيه C واحد و O اتنين: 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. Show the working: number of atoms × atomic mass, then add. Write the unit g/mol. You will use molar mass to find the percentage of iron in an ore. Back to the hook: can you explain why one mole of $\ce{Fe2O3}$ has a mass of 160 g? $\ce{Fe2O3}$ 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.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-stoich.molar-mass