STUDENT SPACE · TOPIC 4
Composition and purity
Calculate percentage composition and purity, then choose the correct mass in a reaction.
The essentials
- Find the mass contribution of the chosen element in the formula.
- Divide by the total relative mass and multiply by 100.
- Ca contributes 40 to a total of 100.
- Percentage of Ca = (40 ÷ 100) × 100 = 40%.
Key vocabulary
| English | Chinese |
|---|---|
| percentage composition | 质量分数 |
| purity | 纯度 |
| impurity | 杂质 |
Quick practice
Does 80% purity mean 80 g of pure substance in every sample?
Need a hint?
Use the relationships above and check particle ratios and units.
Check your reasoning after trying
No. It means 80 g in each 100 g of sample, or a pure-mass fraction of 0.80.
Use Lesson 4 in your worksheet for written practice. Your teacher will discuss those answers in class.