# Mass percentage of an element

> How much of an ore is iron?


## Objectives

- Calculate the mass percentage of iron in an iron compound.
- Explain why an ore has a lower iron percentage than its pure compound.

## 1. Iron ores and extraction of iron




## 2. Which oxide is richer in iron?

Predict: which has the higher percentage of iron by mass?

- Equal: both are just iron and oxygen
- $\ce{Fe3O4}$ ✓
- $\ce{Fe2O3}$

$\ce{Fe3O4}$ has 3 Fe to 4 O (0.75 per O); $\ce{Fe2O3}$ has 2 Fe to 3 O (about 0.67 per O). More iron per oxygen gives $\ce{Fe3O4}$ the higher iron percentage: 72.4% against 70.0%.

[ar-EG] الـ $\ce{Fe3O4}$ فيه 3 Fe قصاد كل 4 O (يعني 0.75 لكل O)، والـ $\ce{Fe2O3}$ فيه 2 Fe قصاد كل 3 O (حوالي 0.67 لكل O). الـ iron الأكتر بالنسبة للـ oxygen بيدّي الـ $\ce{Fe3O4}$ نسبة iron أعلى: 72.4% قصاد 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. Mass of the element = number of its atoms × its atomic mass.


## 4. Worked example: magnetite

Rounded atomic masses used in exam problems Molar mass of $\ce{Fe3O4}$ = 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 O atoms Mass of Fe % Fe


## 5. Your turn: siderite

Pure $\ce{Fe3O4}$ is 72.4% iron. The book's table gives magnetite ore 45–70% iron. Why is the ore lower?

- In the ground, the formula of $\ce{Fe3O4}$ changes.
- The percentage formula only works for pure metals.
- The ore holds impurities. They add mass but no iron. ✓
- Some iron atoms are lost when the ore is weighed.

Magnetite ore is $\ce{Fe3O4}$ 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%.

[ar-EG] الـ magnetite ore هو $\ce{Fe3O4}$ متخلط بشوائب. الشوائب بتزوّد الكتلة من غير ما تزوّد iron، فنفس الـ iron نصيبه بيقل من الكتلة الكلية: 45–70% بدل 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. Write the formula first, then substitute. Give the answer as a percentage. Roasting siderite drives off $\ce{CO2}$, so the ore's iron percentage rises overall. Back to the hook: can you explain why $\ce{Fe3O4}$ has a larger share of iron than $\ce{Fe2O3}$? $\ce{Fe3O4}$ has 3 Fe for 4 O (0.75 per O); $\ce{Fe2O3}$ has 2 Fe for 3 O (about 0.67 per O). More iron per oxygen gives the larger share: 72.4% against 70.0%.



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